Oscilloscopes - what specs to look for?

Again, nothing at all wrong with a good scope, but if I was doing this from your starting point, I would suggest you look at something a little more specialized, like this:

https://quantasylum.com/products/qa401-audio-analyzer

I have no financial interest, just that after being in the hobby for a while, I want more audio specific tools the scopes never gave me. In the old days a scope and audio analyzer and signal generator were 3 different pieces of gear, now I’d rather forego that all and have one.

You may also want to go look at Audio Science Review were they use several bits of kit to analyze audio gear, see what they do.  Oh, also, if you want to play with tubes, make sure you have gear that has probes for the high voltages. :)

Please suit yourself.


Best,


E

eyrepm

Just get this 100MHz dual trace Tektronix, it’s calibrated with probes, and it’s got a digital "frequency counter", and "volt meter" in it as well, you can download the instruction book on line. I had one for years, very reliable, wish I still had it, as I really don’t like the new digital Chinese produced crap.
The seller has 100% good feedback.
All you need is then is a audio frequency generator and you’ve got the works for a cheap test bench. That you won’t have to update.
https://www.ebay.com/itm/Tektronix-2236-100MHz-Oscilloscope-w-counter-timer-DMM-Calibrated-Two-Probes/153883356962?epid=2253449959&hash=item23d429af22:g:754AAOSwpnpeg4MD#viTabs_0

Cheers George

Here’s a link to a Tektronix CFG250 audio frequency generator, a great little unit I also used to have.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR1.TRC0.A0.H0.XTektronix+CFG250.TRS0&_nkw=Tektronix+CFG250&_sacat=0
  
For $400 you've got the lot then DMM, Frequency Counter, 100MHz dual trace scope and a frequency generator.  
All quality Tektronix gear.
 
Cheers George

Actually the same guy with the Tek scope also has a couple of good frequency generators, including a Tek CFG250, the B&K one in nice.
https://www.ebay.com/sch/m.html?_odkw=&epid=2253449959&_ssn=shorewa8&hash=item23d429af22%3Ag%3A754AAOSwpnpeg4MD&item=153883356962&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.TRS0&_nkw=Generator&_sacat=0

Cheers George

Noise and bandwidth are directly linked in most modern scopes. The higher bandwidth ADCs will also have more dynamic range when used at lower sample rates. This is probably the biggest price differentiator as well, as the cost of the ADC is usually the most significant parts cost of the scope. 

I was recently in the market for a scope to use for DIY audio purposes and decided to go for the Siglent SDS1204X-E. This is a 4-channel 200Mhz scope and cost about $750. This might be a little overkill, but offers a lot of nice features including FFT functions for distortion analysis. 

I started my career doing electronics design 40+ years ago, but it's been quite a while since I did any hands-on work, or bought a scope for that matter. The capability of the sub $1K scopes is mind-boggling compared to what I used to spend 10X+ that amount on a few decades ago.