EMG 707 in RG7421?
I can't tell you this from experience, but using an EMG 707, (in either the 74's or 76's) will give you plenty of low-end. But will still give you that searing topend EMG's are known for. The mids are also scrooped to give a very crunchy sound. Is your 6-string trem-equipped or is it a string-thru? When I had a RG7620, I loved the tone, and I had no problem with the low-end. But when I switched exclusively to fixed-bridge guitars (My RG7421 and AX7521) I noticed they have a LOT more low end. but I love it. they also sound a lot tighter. BUT, one thing that I think the 7620 had over the fixed bridges was that the 7620 was a lot more acoustic sounding (not just unplugged, also when played through the amp) and Ive been told was because of the trem-cavity which made the sound resonate much better. Whereas my fixed bridges just have a solid body.
little advice tho, even tho EVERYone says how awesome the 707's are, try one before you buy it. I know this might be near impossible, I've never even seen one in person before!! but try. with the extra routing, it would blow to have to stay with something you dont like.