If your amp has no technical issue, if it's just a question of taste, here is my humble suggestion : change the cab.
The Multiwatts has less bass than the 2 channels Rev G witch is the best for the fat low end. The multiwatts gives the tigh low end. Don't use the Recto cab here but a cab with front loaded speakers as Diezel made them for example. A Matamp or an Orange cab loaded with V30 or Heritage loudspeakers will give you a more balanced tone if you like it dark.
When I play a Multiwatts, I need a darker cab. The low end on the Multiwatt is pure **** ! It gives an articulate sound for djent, 7 or 8 strings guitars. It's not made for doom...
With the Rev D, F and G, I use Bogner 412 V30 cab for the shred, Engl Richie Blackmore 412 V30 cab for thrash, Friedman 412 cab (2 Greenbacks + 2 V30) for the classic rock. They are better than the Mesa cabs (I own 2 of them for others amps). The cab act like an natural EQ. This is probably the best way to fix your issue. I would not play the Greenback speakers here.
Every Recto has it's own tone, they all sound different. I own 3 Ref F and they have not the same sound at all. Maybe yours suffer a lack of bass...
Last but not least. Don't play EL34s, it cuts the low end. The 6L6 is a must have to get the tsunami sound :