From the sound of it, you want a good Marshall and a Rectifier is about as far away from a Marshall as you can get. Modifying one to sound like a Marshall is, in my opinion, a bit of a fools errand.
Embrace it for what it is, and if that doesn't work for you, then you really should leave it alone and get a Marshall.
Some suggestions...I don't know what tubes you have in it, but Doug's Tubes has a tube "kit" for this amp that just kills. That's alot of jack to drop on an amp that you already don't love the sound of, but it will definitely make the amp breathe more and get some more dynamics to it. I picked up a 3 Channel Rectifier for myself a couple of years ago and was astonished at the difference the tubes made. I suggested the same tube setup for a buddy of mine, and he really liked the difference the tubes made in his Rectifier as well. Note...We both like the Rectifier.
As far as the amp itself. If you want Marshall, and assuming a 3 channel version, Ch2, Vintage is where you want to be. Keep the presence down (9 o'clock), tone controls at 5, and on the back turn the loop off (the master volume and solo functions will stop working, use the CH volume for volume control), set the rectifier type to "tube," and power to "spongy." Adjust tone controls to taste, watch the presence control.
If you can bear it (which I doubt), the amp will get less bassy/fizzy and more mid heavy when the volume gets up around noon, but that's definitely NOT a bedroom setting.
This (in my opinion) is about as Marshall as a Rectifier can get. If that does not work for you...Please...Don't mod the amp and go get a Marshall. Even a JCM 900 will be miles closer to the sound you want than a Rectifier will ever get.
I personally do not like EL34's in Recto's, but those tubes will have less bass and more mids to them. When I bought the Doug's Tubes Recto kit, I bought both the EL34's and 6L6's. I took the EL34's for a spin...But that took the amp in a direction I didn't want it to go. You might really like that option though.