Tube Tester

My main goal would be if I hear sonic degradation to determine the bad tube or tubes.

If you think you hear sonic degradation first swap power tubes side to side. It can’t be that all four degraded identically the same so it should sound different and you should know which pair and now just need to narrow it down to which one. Repeat the swap only this time one tube. Now you know which one it is. Unless there is no difference. In that case you know it was all in your mind. There never was a problem to begin with.

Unless its one of the input tubes. In that case swap those. Now this was a very low probability to begin with but at least now you know its not that. Finally you are down to the rectifier tube, which almost certainly its not.

Now the great thing at this point is when you realize you could buy a full set of tubes, all of them, and nice ones too, for much less than the cost of a pretty crappy tube tester. Which is why in 30 years I have never bought a tube tester.

Notice I’m not the only one. I’m with tablejockey on this. Honestly I think if you read what you wrote you will realize you are too.

my first choice in sound but had not wanted to deal with the extra upkeep

So all you do, realize the "extra upkeep" is imaginary. An old Jedi mind trick. These are not the upkeeps you’re looking for.

You don't need one. 

Nobody "needs" a tube tester. As mentioned above, there are ways to get around owning one.
But, I have to say owning one is a pleasure. I happen to have one that has worked flawlessly for many, many years. Yeah, a couple calibrations by Daniel Nelson over the years were costly, but heck, I look at it like a part of my system.
 My system wasn't sounding as good as it should have a short while ago. The second tube I tested in my pre amp was right around the minimum reading. Changed it out  (actually I changed out the pair) and whallah, system sounded better than ever.
Also good for when buying tubes that are advertised with test results, you can compare. And I have had a couple tubes delivered to me testing bad right out of the box (one from a very reputable dealer, who challenged me at first, then admitted it was bad once he got it back). 

I don't think you really need a tester.  I own an Amplitrex, but, it is on loan to a friend who owns many more amps than I do, does repair work on the side for fun and can put that beast to better use.  Others, above, have given great advice about swapping tubes to determine which one, if any, are at fault, and good advice on buying reserve tubes; if something sounds off, just switch in reserve tubes to see if that resolves the problem.