The amp sounds great with the stock Shuguang tubes. I'm not sure that changing the power tubes will improve the sound, however, you'll notice a change if you swap out the preamp tubes, and in my case, not for the better. I tried Mullard EF95's and all it did was increase the gain. I also tried GE 5654W's and they were microphonic. Went back to the stock 6J1's.
Good to know! I wouldn't want to mess with the sound and be unhappy. I'm really pleased as it is. How long have you had yours? Any issues?