I have the p-1 amp with the same tube, so far I have installed the irl530s the lm317a and nichicon audio grade caps. The output caps help extend the bass. I only used the 5814 tube the first night and haven't touched it since. If your up for some tube rolling, you'll be surprised at the differences they can make.
This is my first amp and decent headphones, so I wasn't sure if I would even be able to hear any differences. But after about two months now I have 4 sets of headphones and a pair of senngrados I put together, and can easily tell the differences the tubes can make.
For tubes I have:
A Amperex bugle Boy, it gives a a richer fuller sound with better base
A RCA clear top which is very clean and clear, low bass
A RCA long black plate ( organ pull ) a lot like the bugle boy but slightly cleaner
A tung-sol black glass with the black plates, the bass is heavy and cuts back on the highs
I also have about a dozen others now but the above have become my goto tubes.
The 6 volt tubes have a little more buzz to them whereas the 12 volt tubes seem to run a bit cleaner, but at volume you really can't tell. You will find that certain tubes will fit better with certain headphones and types of music.
The little bear may not be the best, but it's been a lot of fun to start out with. And if your up for some tube rolling I think you'll really enjoy the experience. Beware though, it's like a gate way drug, or lays chips, once you start it's hard to stop.