Re: Why??(Marshall issues)
IME, 75s are dark and/or compressed compared to say a Greenback or G12H, or similar. V30s are bright and a little compressed.
The 75s have a very sinister almost murky sound, without alot of in your face mids...they have a kind of a low mid spike, and a treble spike which gives it some chunk and cut...but theres a big gap inbetween.. They were voiced with 800s in mind, and really IMO that is the amp they sound best with. Though I do like a plexi type with them as well.
V30s IMO are best suited to older plexi-like Marshalls as they add some agressiveness. I dont care for them in many other marshalls, cos JMP and later marshall amps are voiced more agressively in the mids than Plexis to begin with. Its just too nasty an grinding a sound IMO,and thats why the 75s fit in with the later marshalls better (IMO)
If your amp is SS, chances are the speakers were built around that amp for the best voicing/tone. Otherwise, they'd just throw some 75s or V30s or whatnot in the cabs, as it would be cheaper.
Sounds to me like the 75s are doing what they should tonally, but are prolly not a good match for your amp.