Understanding phono stages/What to look for?

The OM20 cartridge you are using is not really suitable for the Rega arm.The Dynavector will be a much better match and then you will begin to hear what your turntable can do.The phono stage you have is very good but obviously the more money you put into something like a phono stage the better it should get.As it stands I think you will like what you hear.Look through the archives here and you will glean much info on phono stages and matching different components.

The PWX power supply will transform your cubes into monsters for a very resonable price. I and a friend each had BCs with the power supply upgrade with great success (I also used a Dyna cart- you'll like that too). You have to spend A LOT MORE $$ to get appreciable gains over the Black Cube with the PWX power supply

Stay with the Lehmann. Quiet, clear, dynamic, handles any cartridge. I kept the Cube in rotation with several high-buck phono stages over the years (Herron, cj premier 15, Xono) and it never embarrased itself. Often I preferred its deep tight bass and black background to the tubed units. Still have a PWX version (I cannot say it is THAT much better than the wall-wart version) which has finally been humbled by my current Groove and Ear 324. I haven't looked around at lower-priced phono stages for 10 years or so, but I do believe the Cube is one of the great values in audio.

Many of the low end SS phono stages sounded more compressed than the tube gear version. BC isn't that great when it was compared with all my full function tube preamps. It is nice and quiet but very harsh and distorted highs. Even the the bass got beaten by some vintage phono sections in both MM and MC. It was no better than the cheap SS phono section either ( except it has MC.) However, it is usable for less $200 in used market. Nothing to write home about it and don't expect it to out beat a $400 CD player either.
You should go through many of the threads here about amplication for the cartidge.

Musical Fidelity makes a nice tube phone stage for about $400. If can't find a local dealer you can order it from Music Direct which has a no questions asked 30 day money back guarantee.