5150s were designed for a PEICE OF CRAP $180 guitar that EVH put together
so really the crappier guitar you have the better the 5150 sounds
the better the guitar... in some ways the worse it sounds...
i think the results you have are normal...
Lol... the oddest logic ever. The 5150 amp was released in 1991, by that time he was already using Ernie Ball guitars. I think the first EB VH guitar came out in 1990. The new guitars and amps came out about the same time and he had been working on them for a while before they were actually released.
Even so, the last version of his 5150 frankenstrat had a PAF and a Charvel body and a Kramer neck. It wasnt the same $180 guitar he used on VH 1.
As for the amp, the injectors are "Paul Gilbert" pickups and pretty much every Paul Gilbert pickup is very bright. Try backing the presence or treble back a bit and turning up the mids.
Not very much, just a tad. Dont make drastic changes. Make a small change, play with it, leave it and come back the next day to see if you like the tone. The 5150 is a great amp with single coils or humbuckers. You just need to spend some time tweaking it.