#1
I have a Crate V33 head, and a Raven RG60. I plan on getting an Engl E530 and using the Crate as a power amp. I really love the way the EL84's break up in it. Just messing around, I tried using the preamp out of the Raven into the Power amp in of the Crate. There is a very loud hum doing this. The guitar signal comes through fine.
The hum is the same as when plugging a cable into the power return of the crate, with the other end of the cable not plugged in to anything at all. Rereading that, it might be hard to follow.

NOTHING AT ALL>>>Crate FX Return
Guitar>>>Raven>>>FX Send>>>Crate FX Return

^Those two combinations sound exactly the same, like the sleeve of the cable isn't making contact inside the Raven.

I've used both effects loops seperately, and they both work perfectly fine. I've tried different cables, with and without pedals etc. Also going straight from the guitar or even guitar+pedals into the FX return of the Crate does not create any hum.

This isn't a major concern now, though it would be nice to use that combo when I'd like to. My real concern is that the same problem will persist if I get an Engl E530 and use the Crate's power amp.
RG351DX - Bridge Dragonfire Screamer, Mid+Neck Fender Hot Noiseless
Peavey Valveking 112 - Eminence GB128
AMT E1 > Joyo AC Tone > Dan'o EQ > Shimverb > Digidelay
#3
Okay, you were curious, and now I am too. So this is how it was being used:
Guitar>>>Raven>>>FX send>>>Crate FX return

I then thought maybe the FX loops just don't like being used incompletely. So I ran a cable from the Crate FX send to Raven FX return, even though the Raven isn't even hooked to a speaker. This cut the hum by about a third of the original. For no reason whatsoever, I too the plug out of the Crate FX send and just plugged into the normal input. The hum is 90% gone, it's even less than I get from my dirtbox now.
RG351DX - Bridge Dragonfire Screamer, Mid+Neck Fender Hot Noiseless
Peavey Valveking 112 - Eminence GB128
AMT E1 > Joyo AC Tone > Dan'o EQ > Shimverb > Digidelay