Based on the specs I am seeing online for the speaker, I think your bottleneck is the interface between the speakers and the amp. You are making the amp do a lot of work to drive the speakers to a reasonable volume. The speakers are 4 ohms, which increases distortion (in all amps, not just tube amps) and will also reduce power and bandwidth on account of the output transformer in the amp.
I think you would be a lot better off with a speaker that is more efficient and higher impedance. You can do that without sacrificing sound quality. A good example would be a set of Dreammaker 2s from Audiokinesis. They are 16 ohms, and about 95 db, meaning that you would need about 1/8th the power or so to make the same sound pressure, and the Dreammakers go just as low if not lower.
I would give this consideration. You can make incremental improvement by replacing the preamp, but you can have transformation by replacing the speakers. I am not saying the speakers are bad BTW, just that your amplifier does not match well with them.
I think you would be a lot better off with a speaker that is more efficient and higher impedance. You can do that without sacrificing sound quality. A good example would be a set of Dreammaker 2s from Audiokinesis. They are 16 ohms, and about 95 db, meaning that you would need about 1/8th the power or so to make the same sound pressure, and the Dreammakers go just as low if not lower.
I would give this consideration. You can make incremental improvement by replacing the preamp, but you can have transformation by replacing the speakers. I am not saying the speakers are bad BTW, just that your amplifier does not match well with them.