Responses from hk_fan

Best 2 ch. Amp, for music, under $1k used
Another vote for McCormack amplifiers. 
Are marantz ma500 monoblocks better than an adcom?
I'm not saying the MA500's are bad, they aren't. They were very good for their price point, however so is your Adcom from what I remember.If you bi-amp then I suspect you will see an improvement. You should also try bridging a pair of the MA500's ... 
Are marantz ma500 monoblocks better than an adcom?
I've owned MA500's and I doubt they will be a big improvement over your Adcom.If they are local, give them a try and hear for yourself, maybe you'll like them. 
Build a new system around pair of Snell Alll's?
Make sure the JC1's you audition are well broken in. They have a notoriously long break-in period. 
Build a new system around pair of Snell Alll's?
I would also recommend you consider the Parasound JC 1s for your Snells. I found nothing near their price point which came close when I was shopping. Honestly, I found nothing I liked better at any price of what I was able to listen to. 
Tube Pre/Monoblock Recommendation
In my opinion if you own Merlins you really should pair them with Joule Electronics. That's what you will find being displayed at EVERY show and the sound is simply outstanding. 
Power Amp advice
I use Parasound JC1's with a ML 326. Fantastic combination.Of those you listed the only one I'd consider would be the Ayre, but I'd still choose my JC1's over it. 
What is the best sounding DAC at $2000-$4000?
The Ayre QB-9 is an excellent choice and fits within your budget. 
Amp Advice : to Proceed or not to Proceed ?
If you can buy the HPA 2 for $500-$800, buy it! Nothing else will come close for that price. 
Need at least 900wpc in 4ohm
Parasound JC1 testing from Stereophile:With continuous drive, the Parasound clipped at 545W into 8 ohms (27.4dBW)¬óway above the specified 400W. ("Clipping" is defined, as usual, as the power level where the measured THD figure reaches 1%, and is s...