V30s are good even-sounding speakers to me. Nice hi, lo, and mid.
75's are very drop-tune metal sounding to me. They have a lot of low end, and not much searing gain on solos.
There are a lot of options. Personally, I'd point the finger at the 75s, but you have your reasons for ditching the V30s.
If I kept the 75's, I'd probably use something more high end and either go honky or piercing to really contrast those 75's. My first choice would be Super Lead 80. That gives you a ton of sparkle on cleans and a lot of sear on solos. But something like a tonker or a tone tubby would round out the tone with more of a vintage snarl. If you like the 75's, swamp thang is a good compliment to give you a swamp box with enough mids to tear up high gain solos.
It's hard to eq down your speakers. That's one of the things I like about having a good pair. You have to play into their strengths more than fight against them. I haven't heard the H30's. I have a Century G12 and a Vintage G12. Does H30 have the heavy magnet? Cause I love heavy magnets. That may be something to look at. Okay, G12H is the heavy magnet.
G12 Heritage is the 65W medium magnet. Not sure which you meant.
It sounds like you would most likely want something with a lot of low end, but more growl than the 75's. That's the big hang up about 75's for me, they don't sing out on leads. Maybe something with good mids but not as boggy as a swamp thang. There is a cool little side-by-side at youtube I'll point you to. Sometime when I'm not at work.