How good is the BAT VK40?

I had one and sold it after a year or two. It had many great features but the for me the sound was a bit uninvolving and a little too "smooth". It did have a sound similar to the tubed VK-30 I had before that in that it did add a thickness or warmth to the midrange and had a great soundstage. I went back to my old Rotel RC-1090 and am happier because it sounds more immediate in the treble and has a sharper edge on bass transients on electronic music, etc. It might be a great match with your Nuforce amps, though. It all comes down to that darn synergy thing. I do miss the great volume control and adjustable balance on the VK-40.

Bizango, thanks for answering. Yes, synergy is really important, I feel the preamp is the weak/missing link in my set up.

I've heard mixed opinion. Some said vk40 is superior to vk30 or even vk31, and vice versa. I see one sold in ebay at $1700 and one advertised in agon at $2000. I guess the price is not that bad.
I am planning to get vk-30 with phono soon to match with vk-200. I chose vk-30 over vk-40 because I'd like to have tube sound in my system, and also due to the price.
I am not sure how well Nuforce Ref9 would work with vk-40.