HD150 Pots dual? OP amps??? (with pics)

hi people,
i got this beautiful MM HD-150 Head that we want to use as a bass head with an ampeg svt-410hlf.
"out of the box" the head sounds great for bass with the ampeg cap. but it lacks a bit in the headroom department and there's a serious hiss issue.
some of the pots need to be replaced urgently. the shaft can be turned endlessly on some.
i'll swap out the elec caps of couse and i'll get spare op amps and see if the hiss issue is caused by them.
what i noticed when reading the schematic is that the schematic (2100-150) states dual pots for Vol pots on both channels. my amp has normal single channel pots (see pics below) that definitely need to be replaced. so what am i going to do? why does it have single channel pots instead of dual ones? maybe i'm just a total noob that can't read schematics? or maybe the head was seriously modded?
i also couldn't find any 2k CTS pots for the midrange control....are they just not available anymore or should i just replace them with 5k ones for more flexibility?
also, as you can see in the pics, all op amps in my head are 1458 type ones while the schematic clearly states that only one 1458 is used and that the rest are TL072........anyone?!
while we're at it...can you spot any other noteworthy things in the pics? any help and comment will be appreciated.
you can view the gutshots of the beatuy here:http://picasaweb.google.com/useme2305/Mu sicManHDOneFifty#

one more question:
the two huge Elec Caps on the bottom of the chassis say 100MFD 450VDC. i suppose i have to swap them out with 450V ones, right? 500V seem to be much more common at the online stores i've browsed...can those be used as replacements or do i have to stricktly use 450V caps?

You can use 500 volt caps. Just never go LOWER. SINGLE pots? HMMM. Don't know what the hell that's all about. I looked at the schem and it SHOULD have ganged pots. Not really sure what that's all about unless it's REALLY been worked on. Matbe somebody tried to fenderize it or something.. I just looked at your pics though and that stuff LOOKS factory. HMMM. I have NOT worked on ANY 150's so hopefully somebody else HAS. Mike.

woah, now i'm confused.
i tried to trace the circuit step by step today to find out if any components were replaced with stuff that doesn't belong in there. my suspicions were caused by the non-dual volume pots on both channels that looked "factory".
well, guess what...i noticed that the entire power amp section is built around EL34s instead of 6L6GCs. (as you might have noticed from the pics from my original post)
so i thought, why the heck would anyone mod this amp so heavily turning the entire thing to be driven by EL34???.....turns out it's all original.
the damn thing even has the tubesheet on the inside of the headshell that states EL34 belong in there. i'll post pics tomorrow of that.
the amp was put together on april 11th 1980 (see pics) while the schematic i downloaded for the HD-150 from this site was penned on 7-8-80.
i guess it's all indeed "factory" and it's just a very early model.
even the output transformer is the 150W version.
i'm very confused.

There were 100W MM amps with 2xEL-34 so why not a 150W amp with 4xEL-34? You are right, it looks like a transition model. I have heard of black label Seventy-Five amps with two EL-34 tubes and your amp seems to fit that picture.
Fix it up, play it and enjoy it. Good score!
-Lars Verholt