Have i reached the limit or do i get a new preamp

I set out a few months ago to imprvove my humble system. Plan was to use my NAD c352 as a power amp and buy a preamp (under $900) to improve soundstage and dynamics. Instead i got a great deal on a b&k ref 2220 power amp in mint condition. Couldn't pass it up, and now i am using the NAD as a preamp. To my ears, a huge improvement. But the question lingers, is there a way to improve my system one last notch. I do think i am that close to squeezing everything out that i can: Cyrus 8x cd with separate power supply. Brass feet on maple block . Diamondback and hero interconnects. Tetra 120u speakers with bedrock cables and lead filled sanus stands. Pangea power cords. Great sound. But is there more with a different preamp, or do i start over at the next level (not gonna do it). Thanks in advance.
Also, i have a little Velodyne sub.
Hell yeah your current preamp is a blockage you NEED to fix in relation to the rest of your system.
Most all the rest is way better than the NAD.
As for finding the right sort of preamp..
under $900.
Right now on Agon the Quicksilver in tube preamps would be My first choice for you from what is currently available.
Another choiice is the Conrad Johnson PV10B A little warmer sound.
Anyway hope you find a good preamp for your setup!
I totally agree with Elizabeth a good preamp will do wonders for you system. Tubes are totally the way to go with a SS amp like the B&K. Quicksilver is a good call, not a C-J fan myself but plenty of affordable options are out there. Good luck and happy listening.
I somewhat agree with Elizabeth's recommendation, however, a better choice would be the Quicksilver Full function preamp. Although you may not need the phono stage this preamp has a low 12.5 ohm output impedance making it compatible with vertually any amplifier and it is not sensitive to cable length.
Tomas - I highly recommend looking into passive preamps, which will cost you probably half of your allotted price but far exceed active preamps at any reasonable price. The B&K amp you have (I have an EX-442 in my second system, which is a good piece of equipment) has the following specs that are important in this decision: 1.4 volt input sensitivity (which is terrific) and 33.2k ohm input impedance, which is on the lower end, so you need to be careful about which passive you buy.

At this point, you are probably throwing up your hands at having to look at these specs, but trust me, looking into this area will allow you to easily buy a unit in your budget which slaughters any preamp under $1k. I mean slaughters. There's no comparison in this price range. Good luck.
Thought I'd close this out. It's been several months ago, I found a good deal on an Anthem AVM20 processors and I couldn't be happier with the improvement in sound. It's five channel and without HDMI, but perfect for my 2.1 set up.

It gives me bass management and I can vary the crossover for the signals that go to my monitors. Taking the digital signal from my Cyrus works better than letting the Cyrus convert to analog, which was a surprise but confirmed by a few blind tests. All of this, as you all predicted, is a distinct class step up from the NAD. System is really sounding great, if I must say so myself.