I have a Marshall 4x12 top cab that I currently have loaded with 4 HM75's. I like the sound, but I could really use a little more bark and high end sizzle (without losing my low end by getting a Greenback clone.
Will pairing a set of Retro 30's with 2 HM75's get me there?
The Ret 30's would be EXACTLY what I would recommend for you ... just like you have stated, they will add "a little more bark and high end sizzle (without losing my low end by getting a Greenback clone)".
You are on the right path.