Old Amp vs. New Amp

some newer amps have newer circuit designs - e.g. for SS, the rise of MOSFETs (some time ago); Class D (nad M22 and others) and the Benchmark (one of the newest designs I've seen)

for tubes, ARC continues to refine their sound & maybe others have too

whatever you have now, I'd buy something I could listen to for 2 to 4 weeks and return 

start working on a list of test music now

your amp exists to drive your speakers - what are they?  and what is their impedance vs. freq. curve like??

The "Magazines" seem to indicate that the "New" version is better than the "Old" version, but I'm not sure if I'm buying it.

That is their job, pushing new merchandise. Audio magazines are part of the audio industry, just another arm of marketing.

That said, older amps are probably just as good, if not abetter value for the $$$ than newer amps. As long as you realize that there is a little bit of maintenance that comes with older amps. Generally speaking, the main power supply capacitors should be checked and/or replaced every 15-30 years, depending on how hot the amp runs and usage.
As stated above, tubes will also need to be replaced as needed.

The speakers I have now are Klipsch KG2's.  A fairly moderate speaker from about 1990.  They sound pretty good though and are rather efficient (I believe).  I am in the process of building some new speakers that will hopefully be done soon.  The new ones are 2way stand speakers using Scanspeak drivers, they will also be fairly efficient and are 4ohm.  Once I finish those, I will be starting on some more elaborate 3way floor standing speakers with dual woofers.  So I am looking for an amp suitable for those, not so much what I have now.

How do you know when the capacitors are in need of replacing?  Is it subtle loss of fidelity?  Or more of a drastic drop off?

I've experienced bad tubes (in guitar amps) that is a fairly drastic drop off at least mine was.

petrela,

     I think you need to make a distinction between the older tube and class A amps and the older class A/B amps.     

     Good quality older tube and class A amps would likely offer the best bargains.  These amps offer great sound quality at reduced prices as long as you're aware that some of them may require extra expenses to keep them functioning at optimum levels.

     The performance of good quality older class A/B amps, however, are equaled or bettered by numerous modern amps using newer technology.  In my experience, there are absolutely no advantages to be gained by using class A/B amps rather than good class D amps.  
     
      Older class A/B amps can be bought at lower prices but the true bargains are the numerous reasonably priced class D amps that currently outperform them on all important audio criteria while also being a fraction of the size and weight, more efficient and cooler running.

     I honestly cannot think of a single advantage you would gain by buying an older class A/B amp.  Unless your budget is $10k or more, I believe you'd be better served eliminating all class A/B amps, older and current models, from consideration.

Just my opinion based on experience,
   Tim

Thanks for your input.  Honestly I am a little skeptical about class D.  I don't really have a reason to be though.

I believe for the most part the ones I am looking at are class A at least to a certain point.  It's a little hard to tell sometimes.  The specs aren't always very clear about that.

I am kind of leaning towards some of the older Mark Levinson models (300 series mainly).