Best preamp/ Amp combo or intgrated amp??

Sorry for the late response but just came across this thread again. Your infinity speakers may be fine. Even if I knew what model you had only you can tell their sound quality since you can hear their acoustic properties in your room. If you want to compare sound quality vs other speakers with similar electronics running them in a similar room set up, you will have to judge, as we all do, according to personal tastes....

Are you saying that my infinty speakers are no good 55dok?

If you are buying a new amp, I would agree with the recommendation of the NAD C320BEE, which has an excellent reputation. Other choices might include the Cambridge Audio Azur 340A (40 w/c), 540A (50 w/c) or 640A (65 w/c) amps available from www.audioadvisor.com. If you are buying used, which can offer better value for money, I would suggest considering any one of the NAD integrated amps in the 40 to 80 watt range offered during the past few years -- the C320, C340, C350 or C352, for example. (I have owned a C340 for several years.) While available for reasonable prices, these are good-sounding, flexible amplifiers which can grow with your system. They all have preamp outputs, so the preamp section can be used to drive a larger power amplifier if you decide later on that your system needs more power. If you can't wait for more power because you need to drive electrostatic speakers, you might also consider buying an older NAD 3400 integrated (100 w/c), or a combination of a NAD 1300 preamplifier and a NAD 2400 power amplifier (100 w/c), if you can find them used. There is a fairly steady stream of used NAD equipment up for auction on eBay at any given time. These may not be the ultimate amplifiers, but they are a good, cost-effective place to start, and they have a ready resale value if you later decide to trade up.

You might consider a receiver instead of an integrated amp. You can purchase a used luxman R-117 receiver for approx. $280 on Ebay (from a high ranked seller) when they become available. They have 160 watts/channel and have a cd bypass switch and can take a MC cartridge. The nakamichi stasis SR-3 is a sweeter sounding receiver but only 80 watts/channel and can be purchased for $150-200. I would then spend the rest on speakers. If you have the room you could get used magnepan 1.6 QRs for $750-950.

Brand new Marantz PM7200 is worth considering.

I have no knowledge of Infinity electro stats but, it has been posted quite often that in general electro stats are very power hungry. As far as separate pre/power goes I can't think of any that are in the $500 range. I would suggest the NAD 320BE integrated that retails for $399. I wouldn't think this would be enough for the Infinity's so, go to your local Hi-Fi stores, not the big retailers, and ask for help. You will find that most hi-fi shops will help you get the most for your budget and will account for all of the factors that help and hurt a system, unlike the box pushers. Listen to as many different models as you can in your price range then make your decision. In the end, buy what sounds best to you.