Recommendations for a new integrated amp for Vandersteen 3a speakers

I would vote for the Musical Fidelity as I know it pairs well with Vandersteen speakers.  If you can find an M6 at that price, excellent as they tend to sell used for a bit above that.  The M5 at 150w would work and I am sure that you could get to your target price either via haggling or trade in on new gear.  

Full disclosure, I am an MF dealer and am a bit biased.  That being said, a user out on stereophile has had an MF M6PRX amp paired with the Treo's and has raved about it for years.  MF and Vandersteen work very well together.  

Thanks to everyone for the good advice.  I will definitely consider  finding a pair of 3a sigs as part of the upgrade path that will yield the highest impact.  Also, will try and find a used Belles to consider. As background- I lost out on a bid for a Cronus Magnum to try and was considering a Mcintosh Mc2120 but decided i would rather find something newer in my price range.  I borrowed a friends Bryston 2b to see how it compared to my NAD 2100 and couldn't really tell a difference.  Also i do have a SVS SB-1000 hooked up which definitely enhances the base performance-so any suggestinos for integrateds that can take advantage of that are appreciated.
Lastly, does anyone think i should keep the adcom preamp in the system and just get a better amp? Or will it simply limit the performance of whatever amp i hook it up with?

I’ve used Belles amps with Vandy 1s through quatros, including 3a sigs and think they are a great match as others have stated. I have not heard the aria integrated but have read it is special with Vandersteens. They show up used in your price range. Belles are not flashy, but they sound right. I’ve also tried a number of preamps and they make a huge difference to the sound. I have not heard your adcom, but imagine a newer high quality integrated would outperform an amp mated with your preamp. Ayre and Aesthetix are the other matches you’ve likely read about, but they will be at a higher price point than you are targeting.

I've owned 2 of the integrateds you are looking at.  First was the Vincent.  I liked it but it had problems with the tubes.  German design, chinese build.  Sound was very pleasing.  Laid back presentation.  The music traveled to you it didn't attack. Had tone controls which is rare. So after that I traded to the MF 6si....not 6i.  Overall I like the MF better.  This thing is a beast and will handle anything you throw at it.  As you raise the volume you feel it building power without loss off nuance.  You can play a great classical piece and let flow and right after that smack on some hard rock and it will give you the meat and potatoes without sacrificing detail.  Lastly, I've heard the rogue as well that is a fine choice too.  good luck.

...sell the preamp - go integrated or a power amp that you can play directly. Save money on interconnects - and simply your system...