Try moving your speakers closer to front wall. I think the reason tube amps don't work well with modern speakers is because of smaller enclosures which, for the most part, produce "less" bass. People didn't want two BIG speakers, and I do mean BIG, in their living room when the big change from mono to stereo came along. If you look at old of houses with mono speakers, you'll discover most speakers were shoved against a wall.
Also, you might try "tube rolling."
Also, you might try "tube rolling."