Old Amp vs. New Amp

I should probably qualify my cable comments.  Personally I'd never spend $1000 for a cable.  I currently have 2 pairs of IC's that cost about $300 for both.  My Wireworld speaker cables cost about the same.  These both brought improved sonics for me and I'm happy there.  

As far as amps go, I'd concur that you could get more value in a used one while not losing out on sound quality.  Amps are a mature technology and as long as you get a well built one, I'd suggest you'd be fine.  

The Parasound gear I have has a slightly warm sound, with the top end probably rolled off a little, and I suspect the P5 might have the same characteristics.  I've read that the Nad's sound is similar, so when amp shopping, you may want to avoid amps with a warm sound, and go for neutral or forward sounding instead.  Unless of course you prefer a warmer sound.

That's pretty much what I was thinking on the older amps.  I feel like I would get a better amp for the same money.  As for my P5, I could end up wanting to upgrade that later, but for now, I feel like that is going to be a very good pre for the money.

I agree with your thoughts on the cables.  I think the moderately priced cables are the way to go.  Although I think they are also pretty overpriced.  But I think cables particularly speaker and power cables need to fit the application.  For example, I don't think you would want to use the same cables for a 50wpc amp into bookshelf speakers as you would pushing 1000wpc into giant speakers.

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.