You will not get your money back if you choose to sell your modified Adcom, or most modified equipment. Go with the Stratos.
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Although I don’t find the Adcoms especially bad in any one area I feel there is definitely room for improvement, more liquid mids and softer highs would be great.
I'm sure many will disagree, but what you're asking an amp to do is a characteristic of a speaker. I think you'll have a much greater chance of reaching your goal by changing speakers.
A few years ago I replaced my Adcom GFA 555 II with an Odyssey Stratos Dual Mono. I felt there was an appreciable improvement -- while I'm not always sure of the words to use for such camparisons, the Stratos seemed deeper and darker. (Of course, the Stratos was around 1300 USD used, quite a bit more than a used Adcom or even two.) Furthermore, Klaus at Odyssey is well known around here for being a straight up guy who supports his used equipment, so I suspect you'd not regret your purchase -- I certainly have been very pleased. So, although I'm not familiar with the mods in question, I'd 3rd the suggestion to go with a Stratos.
Worth noting, though, that you're contemplating spending in the neighborhood of 1000 USD, whether you do the mods or the Stratos. As Bob_reynolds says, it is possible that you might notice more improvement if you put the grand elsewhere, especially since you like the Adcoms (I liked mine too). Of course, a used Stratos is likely to resell well, so you could "audition" a used one for relatively low risk, and see what kind of result you get. (Though notice that the Stratos is a beefy piece, and the shipping expense may be non-trivial.)
My 2c. John
sorry for the late reply on this ... can I still make a suggestion?
Swap your speaker cable out. On more than one system (both NAD, actually) more liquid everything and softer highs came when I swapped my home-brew anti-cables with Audioquest Type 4.
I have a 6' pair I can lend you if you want to check them out :) Meanwhile, I'm looking at 545s and 535's for myself, as well as B&K stuff from the same era.
The 545 is a decent amp, but the mark 2 version has the better power supplies and would be worth seeking out over the mark 1 versions.
In either case, both suffer from some grain on the top end. I have worked a number of the Adcom amps; all improve greatly with cap upgrades, bypass caps on the power supply, soft recovery bridges, etc; and in the case of the GFA 555 mk2 and others, tossing out those chrome plated steel buss bars and replacing them with copper or brass makes a huge sonic improvement.
The studio 100s are pretty open and clear on the top and will show any grain coming through from the amp. This can easily translate into listener fatigue. Same can be true with stock Parasound amps and Paradigm speakers.
As another poster pointed out, you can have musical concepts upgrade them for you, but you're likely not to get that money back if you ever sell the amps.
I would suggest doing the cap upgrades yourself; the cost of the parts will be less than 15 bucks. Just will require some of your time to install them.
It appears most of your source material is CD, so any cd graininess is increased additionally as it passes through the amp.
I modified a 545 Mk2 for a friend. Like you, he was interested in replacement options. After the upgrade was complete, he could not have been happier with the sound.
The paradigms are not difficult speakers to drive, so I would think that a pair of modified 545's might really be the ticket.
I will definitely look into changing the cables.
Johnss, I pulled off the lid to one of the Adcoms. There are 4 very large capacitors marked "63v 10000", they are blue in color. I'm assuming these are the ones that need to be replaced?
I was just listening to Elvis Costello and the Allman Brothers Live at Fillmore East. My main qualm is brightness at this point. I feel if I can get the highs under control I'll be happy with the system until budget allows an amplifier upgrade (I just bought my first house and am getting married in 5 weeks so that could be a while, I know, woe is me).