Graham Slee Solo+PSU1 Vs. Lehmann Linear

I am slowly putting together a 'better' headphone based system. At present, I'm using a Graham Slee Reflex phono stage with a Solo (both with the Slee PSU1 power supplies) with a pair of Sennheiser 650 cans. (an Technics SP10 TT is in the house, and soon, an SME 12" arm will be part of the set-up too)

Although I am currently using a Graham Slee Solo with the PSU1 power supply, I think there might be better headphone amps out there- especially as I have my eye on a set of Sennheiser 800 cans... I have noticed that the Solo does struggle occasionally when pressed with very busy passages or a bit of extra gain. (not that I'm listening at 747 sound-levels BTW)

In my $1,250ish price range, I have whittled it down to the Lehmann Black Cube Linear from Germany.

The reviews on the Linear are excellent. However, I'm wondering if the change-over would be a step-up or a step-across? Other options? (I don't really want a tube headphone amp) Any 'Goners had experience with both amps?

Music? 70s to now- rock, pop and dance. I have a large collection of records that includes many dynamic, loudly cut 12" singles... The winner must have a firm grip on the bass (not shy either!) and excellent PRT.

I am also looking at a phono stages, but that's another thread....