Upgrading from Integrated Amp to Separates

Look into the Lyngdorf 3400.  Brand new model. I have the older 2170.  I used it on the 40.1s.  Amazing together. Sell your dac, power cord and ICs as you won’t need them. The room correction in the 3400 is stunning and SOTA. I sold over $18,000 of tube separates and like the 2170 better. The 3400 is even better. This piece will assure the 40.1s sound as the builder intended in your room. I know this is not conventional high end advice. This is not a conventional piece and it breaks all manner of aphile notions and perceptions. No worries regarding tubes as this unit transcends that whole SS vs. tube thing. Just is beyond that. It sounds like music and your room will no longer be against you. It eliminates the room. 

I speak all this based on vast experience. I know, this is not what you wanted based on the subject, but I sense you may welcome it. Enjoy. 

I have the PL Dialogue Premium and went to BAT separates. VK55 amp and VK31SE preamp with great results. Yes they are older but sound fantastic. With your budget I'm sure you can get better.  Grannyring brings up a good suggestion also.  I know another Audiogoner went this Lyngdorf route and is really happy with his choice.  

Another popular choice with the Harbeth speakers are the Luxman integrated amps.

Integrated Amp >>> Separates    Linear Tube Audio MicroZotl Preamplifier + Zotl Ultralinear present an interesting pairing for the Harbeth 40.1s. They come in at budget and keep you in tubes!

Don't forget about Audio Research.  I think they are very nice for a tube amp/preamp choice.  Probably the most transparent "solid state" like tube equipment.  BAT is nice and powerful, but I have read they are dark sounding.  It really depends on what your after.