Ight, the stock tubes are a Sovtek 12ax7 - which is ok and used in the HG 50 from HarpGear. The 6v6 is a generic model made in China. They sound ok and are totally useable for harp. For the $200ish you can get the amp for, you can pretty much plug and play. The amp comes totally useable.
However, for around $30 depending on where you purchase, you can pick up a different preamp tube and 6v6 that will change the tone of the amp. The preamp tube makes the biggest difference. I use a SM 57 for my mic, and with a 12ax7 can easily run the amp at 2-3 o'clock without feedback. This is quite a bit of the amp's volume. The tone control is also useable at this point. For said price, I picked up a Tung Sol RI 12ax7 and JJ 6v6. The tone is clearer, louder, fuller, and the amp has more bite and bass.
Before doing any internal mods or switching the speaker, I suggest rolling preamp tubes to taste. I have tried a 5751 which is pretty nice...it also makes the pull boost useable for harp.