The Solo 50 has a 6L6<->EL34 bias switch, as does the Dual Rec. No idea how it sounds with EL34s, but I would imagine that it would still sound like a Rectifier, just with a bit less low end & more upper mids.
Yes, EL34s clearly sound different to 6L6s, but it's not going to transform it into a Marshall. My Marshall SL-X has 6L6s & sounds nothing like my Rectifier did...