Originally, the first Sunn amplifier was created in the late 60s by a bass player who thought his amp was not powerful enough for large stages. The first model T was marketed as an amp that could suit both guitar and bass, a 150 Watt beast with four 6550 tubes, inspired by the Fender Bassman 5F6A amp. The GuitarPCB “Sunn-T” actually is “bass ready”.
C1 in the GuitarPCB’s “Sunn-T” is just high frequency rejection. C3 & C7 see to the bass-readiness. The only other places where a bass guitar might benefit are at C4 & C5 (both 22nF) and you could easily bump them up to 47nF or 68nF potentially better low frequency pass-thru. (IMO)
Coda-Effects has a good write up and analysis on the Sunn model T here: Sunn-T