If it is a lack of bass impact and not room related suckout, the output tubes made the amps sound as if it gained another 50w of power control and drive over ANY other type output tube, over NOS Gec kt88, Tungsol 6550's.
There is no point taking about tonals of these lusher sounding tubes being better as the bass is the most critical issue you have. Other suggested tweaks will improve resolution, room acoustics but not improving the bass energy which this amp lacks.
The fuses in the amp do not have any audiophile equivalents.