First, welcome to Talkbass! We're mostly nice here.
I've owned and gigged an 803 for about 1.5 years (I lose track), though not exclusively. You'll find my comments praising the amp earlier in this thread. I'm still very fond of it.
It's a good amp for a varying repertoire or multiple genres as it's a real chameleon in the preamp. You can get a whole variety of tone types in analog fashion just using the multiple inputs and voicing switch. Possibly the only voicing that would be optimal through a larger cab might be the qtube voice, which does have some interaction with master volume as I recall. Still, no reason you couldn't use any of the 6 inputs with any cab configuration. I've used them all, and there's not a bad sound in the amp! I expect the q-tube might be the closest to your Fender, but any "boom" is easily controlled with its EQ.
That said, any amp can run a small cab, but it requires real common sense, restraint and good ears. It's not likely any manufacturer would recommend it to you. But I run the 803 into an 8 ohm 112 that can handle 400 watts without any fear, but with care. Always listen to your speaker for signs of distress.
It's hard IMO to expect a 110 box to fill a 600 seat room regardless of the amp (forget about stoner rock), and I'd be concerned about overtaxing it - same with a 2x8, but I've no personal experience with 8's or with GR cabs to advise you. I also haven't heard if you plan to carry the room or have PA support. That figures into your planning big-time. If I were bringing an 803 to a room that size without PA support, I'd have a minimum 210, and an efficient one at that. It's likely obvious to you that you can easily expand your reach with a second 8 ohm cab if and when you need it.
I do think that @gtirard, the OP on this thread, may have some GR experience. I'd look for his input on this. I cant comment on Quilter CS as I've never needed any

.
Again, welcome, and I applaud your taste in amps!