Any mods I can do to a Boss PH-1r?

[Ry]

It would be interesting to know the FET type involved!

They are marked K30AGR.  Each has a nice bit of yellow paint on the top, marking them as matched, I would assume.

Ry

[Vsat]

That would be the 2SK30A-GR....
The 2SK30A-GR and 2SK30A-Y were both common Japanese N-channel FETs. The -Y is still in production. GR now more difficult to find, has a lower pinchoff voltage than the -Y.
Regards, Mike
P.S. "GR" stands for green, "Y" stands for yellow... even though these transistors don't often have colored markings.

[slideman82]

Good info! I really like the 2SK30 sound!