How to clear/reset OSx without upgrading?

If you rely on specific software or hardware and are not an average user that just does a little browsing the web, writing some text and emails, then you are better waiting a while to make major changes of your operating system. You can even wait a couple of days with smaller dot version or security updates and see what other (often more experienced) users will report. If you have a running (production) system, you usually don't touch it. If you have just one Mac that always has to work, you know that every time you modify the system or install new software is a potential risk to lay your productivity down. That's not new and even as an experienced user, you should have a good backup strategy. There are other threads that cover backup topics in depth, but for your question on os updates that means, make a fully functional bootable clone of your system with a backup app you trust on an intact external drive, at least before updating major os versions. Sometimes this can also save your day if you apply minor os updates as you can easily revert to a working state.
I personally think, that the release time of one year for the last major Mac OS X versions is too short to really serve a good quality production proof os. Bugs are not removed as far as they were formerly when there were between 8 and 11 minor updates to the final release. This short release cycle also often leads to software incompatibilities that third party developers, especially for performance/stability critical drivers can't handle so easily.
I would say: No, this fear is not irrationally! But you should do what you can to avoid trouble on your side and that you can revert changes.

Most users I know have installer USB flash drives of the Mac OS X releases since the system has to be downloaded from the Mac App Store. That's not only practical for upgrading to a specific os version, but also for deploying the upgrade to more machines without downloading again and again. The only easy and legal way that I know to upgrade to Yosemite would be to have downloaded a copy of it, before El Capitan was released. If you haven't done that, you could ask in a local Mac Store if they are willing to help you. I can't recommend to ask a friend for a flash drive installer as this is probably not the way Apple wants to license the os to you. To download a copy not only has legal problems but also security risks.
For future releases and likely El Capitan, if you don't want to install that time you can find it on the Mac App Store, wait just to the last minor release, download before the next major release is out, quit the installer, don't install it and make a copy. The last step sounds easy, but you can run into several problems. One route that works is the usage of DiskMakerX to make an USB flash drive installer. So hurry and grab El Capitan, before you can't anymore and good luck with hopefully helpful staff in your local store for Yosemite.

That question is not clear enough to me, but I try to answer! I presume it's an internal SSD with the os on it. Every SSD that I'm reading about, loses performance if full of data and aged. Some more, some less. I guess that's normal and the downside of recent SSDs. What you can do is:
- First backup your (biggest working) files and other stuff like games you don't play so often or better do a full system clone.
- Delete those unneeded files on your SSD.
- Depending on the SSD you are using it could take some time to get a performance gain or there could be no performance boost.
- If you are using a non-Apple SSD you can activate TRIM with a kernel patch in Mavericks or with a simple terminal command in Yosemite. If that's an option for you, you must decide by yourself. You can check if TRIM is already enabled for your drive with: Apple menu -> About this Mac -> more Information -> System report -> Hardware -> SATA/SATAExpress -> choose your SSD in the right part of the window -> search for the info TRIM support. If it says no, then you should think about enabling it. Patching the kernel must be reverted before upgrading to the next os and is not officially supported. There is no guarantee that you see any performance boost with TRIM, but chances are there.
- Run the periodic maintenance scripts for file defragmentation
- Delete user/system/application caches
- If there is a smaller problem with the os, the latest Combo-Update for Mavericks can help you. Just install it over your running system then.
- If you have audio dropouts, it must not have automatically something to do with your hard drive or that your system is broken. You can find more specific posts about this topic. Without knowing anything about your setup and symptoms your Mac is suffering from no one can really help you with that.

 

1. You need to have 'bought' Yosemite on the App store already, and then you can just download the installer. If you haven't, then you have to get someone who has done so, to install it unto a USB stick.
2. Reboot, hold down command+R. Format, reinstall. Remember to backup first!
3. "Crapped out and stopped working" can mean a lot of things. But likely, yes, coincidence, or you repeatedly install something that really messes with the system. Millions of people do this regularly without issues.

 

Yes, that's what he certainly wants! For Yumid it could be also interesting to know, that the Mac OS X Internet Recovery (Option-Command-R) would bring him back the original Mac OS that was preinstalled on his Mac, if not too old. From that os he could start downloading all Mac OS X versions he downloaded including Mavericks. Then make USB installer drives. That way he would be able to downgrade from Yosemite or higher to his working Mavericks. With just a plain install he would risk not being able to downgrade to his working Mavericks later.

 

My thoughts.

You didn't tell us much of anything about the Mac you're using.

What I'd try:

1. Get another SSD. They're cheap now. For the time being, connect it externally via USB3. This will allow you to "prep and test" it BEFORE you commit to making it your primary drive.

2. Install either Yosemite or El Capitan onto it. I'd suggest El Capitan because of recent Mac releases, it seems very stable.
BUT -- you need to get El Capitan QUICKLY, because I sense that it will no longer be available from Apple after Sierra is publicly released on September 20!

3. Install the rest of your apps, accounts, etc. onto the new SSD.

How do things work this way?
Can you run Pro Tools?
Does it run ok?

Once you get it where you want it, then it's time to swap it out to become the new internal boot drive...