Integrated with passive preamp section. Good or bad idea, or both ?

I recently put Burson Soloist preamp/headphone amp between the source and Redgum RGi120 integrated with passive preamp section, and everything is better with perhaps the slightest loss of resolution. And the Redgum doesn't even have power amp in input, I simply plugged additional interconnect into the line in. The sound was already quite good without the Burson, but the improvement was significant.

The preamp section in Gryphon Diablo´s are passive.

It depends on the sensitivity of the amplifier.  If the amplifier is extremely sensitive there is no need for gain from the preamp.

The Art Audio amps like the Diavolo utilize a passive preamp stage when converted to an integrated and our preamps offer switchable gain but when paired with our amps offer 0 dB.  

Alternatively, with a less sensitive amplifier, 6-12dB of gain may be required for it to sound right.