Bigwave; I use an SF Line 2 with a McCormack DNA2 Rev.A (Vand. 5 spkrs.) and don't find the combination soft at all-- laid back yes, but also pretty neutral and dynamic. It's my guess that it's more likely an interconnect or speaker cable issue in your case. Either can have a significant affect on especially a soft versus dynamic presentation. FWIW, I use Syn. Res. Phase 2 (Kaleidoscope from pre-amp to amp) w/active shield ICs, and Syn. Res. Sig. #2 copper cables known for their slight warmth. What else is in your system?
Also, I tried a pair of Seimens tubes in the driver positions of my Line 2 and found the music presentation to be overly big, diffuse, and "bloomy". The stock Sovteks are much more neutal, albeit a bit cool, but in my system other things make up for that. Good Luck. Craig