What kinds of tubes I can use?

Here is a site that describes the presentation you can expect with various 6SN7 tubes. (http://home.att.net/~chimeraone/6sn7sound.html) I've personally tried all the top 6SN7GT/VT-231 variants mentioned, and have found the Sylvania 6SN7W(tall bottle) to have the most accurate bass reproduction and slam(hated the short bottle). The Tung Sol round plate(oval mica/metal supports) is the winner for sound staging width/depth and ambience retrieval. They are both tonally neutral and faithful to the signal. Next for neutrality and sound stage performance are the Ken-Rad VT-231 and Sylvania 6SN7GT(bottom getters from the 40's). There are a variety of good 12AX7 tubes that will sub for what you've got from Telefunken and Siemens. My personal favs are the ECC803S(tele) and E83CC(Siemens). Here's a site that describes the presentation of some of the variants: ( http://www.tubeworld.com/12ax7.htm ). You'll have to be careful/picky shopping for ANY NOS tubes, as the market is flooded with cons. The KT-88 is an easy tube to replace, but not with NOS. The prices are nuts nowdays. Upscale Audio sells the latest iterations of the Winged 'C' KT-88s and 6550s, from the St Petersburg plant, either of which will provide nice bottom and clean mids/highs. Their matching, testing, grading and burn-in procedures are second to none. (http://www.upscaleaudio.com/6550KT88-Types_c_85.html) Of course: good sound staging/imaging is VERY dependant on your speakers, system balance, room acoustics, etc. BUT- you are certain to hear a marked improvement regarding the realism of your presentation with the above mentioned tubes.

I have no idea of the quality of your amp, but- As I stated at the end of my other post; there are numerous things on which good sound staging depends. To be blunt; Coming from China(or anywhere. China just makes it more likely), your amp COULD contain some really lousy parts that represent a disparity from channel to channel, or simply that will not cleanly pass ambience info. Cheap output transformers, or a poorly designed power supply, can drastically/negatively affect sound staging. The World's best tubes won't make a sow's ear into a silk purse, though they COULD make it somewhat more pleasant to listen to. I didn't mention a high quality source(arm/cartridge/table or CDP), preamp, interconnects, speaker cables and vinyl or CDs that were recorded/engineered/pressed well are all very necessary. A serious mismatch in any of the tube pairs that your amp contains can also seriouly degrade staging/imaging. YES- It is a possibilty that the amp is too small for the B&Ws, or the combo too small for the size of the room. Are the SPLs you are reaching satisfying? If they are: power's not the problem. Have you had good sound stage presentation in your listening room, with the rest of the system and those speakers, prior to replacing your power amp? YES- IF everything else is of quality, balanced and set up correctly: good NOS tubes should profoundly affect your reproduction.