Best linestage I've had in my system was the Musical Fidelity kW. You can google the Stereophile review. Uses 6112 tubes, and was the longest lasting component in my system by a long shot. With that preamp, I finally got the combined sonic benefits that had previously been trade-offs between passive (detail) and active (dynamics and oomph) preamps.
Only reason I am not using it now is that I am employing a DEQX as DAC, preamp and room correction. Something like the DEQX might be worth looking into, as it will do a world of good on both your speakers' frequency response and other characteristics as well as correcting room effects. I use the DEQX primarily with vinyl and here no degredation from the ADC that the unit uses with analog inputs. You can check out more discussion of these on the "Is DEQX a game changer?" thread
Only reason I am not using it now is that I am employing a DEQX as DAC, preamp and room correction. Something like the DEQX might be worth looking into, as it will do a world of good on both your speakers' frequency response and other characteristics as well as correcting room effects. I use the DEQX primarily with vinyl and here no degredation from the ADC that the unit uses with analog inputs. You can check out more discussion of these on the "Is DEQX a game changer?" thread