I have a Marshall Valvestate VS100R combo, and very rarely do I have to turn it up past 3. I steadfastly believe that the more controlled your onstage volume is, the easier time your sound guy will have making you sound good at mix position. One thing to consider is sidewashing your guitar amps, leaving the bass amps back by the drums. You can also consider getting an amp stand that angles the sound up.
I agree generally with the comments about not needing the volume at those levels for practice, and also about cabinet separation and maybe angling the cab up at you, or raising it closer to head height. Also agree about perceived volume of valve versus sold-state amps.
That being said, your problem with feedback may very well be due to your pickups. Try testing to see if the feedback problem seems to be related to particular instruments and/or pickup(s). I'm shocked at how many stock pickups on Gibson and Fender and other supposedly high quality guitars are microphonic and/or poorly shielded from picking up stray induction fields from amps, etc. especially in this age of ultra-gain amps and uber-distortion pedals.
I'd suggest either replacing the pickups with high-quality aftermarket pickups, or having a tech "pot" your pickups. The most common cause of microphonic pickups is the windings of the pickup being loose, which happens with age or from poor quality control during the pickup-winding process. Potting corrects this by soaking the windings in either wax or an epoxy compound to prevent the windings from vibrating at high volume levels.
You can also improve shielding by wiring with high quality teflon-sleeved shielded mini cable, and also using copper-foil shielding material which has adhesive on one side to shield the pickup and control cavities. Make sure the foil is tied to ground. If the foil isn't grounded, it won't help.
Hope that helps.
Strat