Pass Labs Preamp with F5 Series Amps??

Some preamp and amp combinations can work in your benefit, some may not.
Depends lot on your speakers.
When i finished crossover on my three domes 4-way speakers, i was using F4 and F5 amps, with various buffers and preamps.
Lots of combinations were tried. Some resulted in soft mellow sound, but lacking in details. Some in sharp edgy sound, but very accurate.
All with the same speakers.
Combo ba3fa and F5 was too sharp but revealing.
Vendetta clone pre and F4 was my favorite.
F5 with tube pre was just right.
As i said, all depends on your speakers.