Ideas on making an RD50 less trebley?

I'm looking for a mod or some suggetions on how to get my RD50 to be less trebley. The treble is set to 0, as well as all the settings on my pedalboard, and I have tried different speakers. Does anyone else have this problem? I have a HD130 and an 130 212 with no problems at all.

I have a 112RD50 as well as a 112RP65. The RD50 does sound a bit thinner than the other higher voltage MM amps, but considering its weight and size, I love mine. But, I understand your problem. There are a few things that can be done. Which speaker does your's have? I now have an Eminence Texas Heat installed, and it sounds great. Adds bottom end and tames the highs. I tried an Eminence Red White and Blues, I found it too bright in this amp. Also tried an EV Force which was WAY too bright. Also tried an Eminence V30 which was OK, but sounded too Celestion, (which is how is supposed to sound) for me.
MM made some mods to the circuits over the 3 years this amp was made - one seems to address this issue. They changed the value of R1. Check out the schematic section.
Also, consider your tubes and condition of your caps. I use winged SED 6L6 which give a smooth solid sound. I think Sovtek tend to be high sounding and not so full. JJ could be an option, but they tend too be cold in this amp since you can't adjust the bias.
Have you considered your filter caps? If your filter caps are bad, which is possible in an amp this age, you may be loosing your low end, which could be your problem.
Another thing I did to mine was to swap the TL072 op amps for JRC 4556. Not a huge difference, but I did notice that it warmed the amp a bit, a small bit. Very small bit.
Lastly, I play a Les Paul with HB pickups and a Godin with Duncan P90's. I suppose a strat or tele might sound pretty thin through this amp. Anyway, that's my .02$

You could try a jensen neo speaker. I have one in an rd50-10, and it's very warm, with hardly any treble when set at '1'. Also, it's very light, and distorts much less than the stock MM speaker!!Mind you, like mike h above, I use a 335 not a fender. my 2p worth!