My CS300XS is completely silent, no hiss nor hum, and a very full and pleasing tone, and this is with the stock tubes. Of course, it sounds even better when I tube roll through my stash of vintage RCA, Mullard, and Philips. I would suggest you have the amp serviced since it most certainly sounds like there is something amiss. Also, I suspect the computer sound card is your tonal bottleneck in that system,
In fact, I have heard many superlatives to describe the Leben sound, but 'thin' is not one of them. :) Best of luck sorting things out.