Tube amp for "difficult" speakers,,,

Unless the speakers have a flat impedance curve no tube amp is going to do them justice for reasons mentioned above by Rlawry.

If you want a refined and liquid solid state sound, consider Pass Labs XA-.5 series amps.

If your speakers do present a difficult load, there is proabably no better all tube amp (hybrids would also work)to deal with dipping impedances the Music Reference RM9 Special Edition which is one of the few amps that actually increases power into 4 ohms (200 watts)compared with 8 ohms (160 watts) and that should be enough power to drive speakers with your sensitivity. However if I did have a difficult speaker load I would proabably look at the Pass XA.5 amps that Tvad recommends.

Increasing power to 200 watts into 4 ohms versus 160 watts into 8 ohms still causes a volume imbalance in the 4 ohm frequencies (they'll have less volume than the higher impedance frequencies). The speakers will sound tipped up with lack of body, IMO.

You need 320 watts into 4 ohms for 8 ohm speakers to produce a balanced sound from top to bottom.

The impedance curve is key. If it's relatively flat, you're in good shape to use a tube amp.

Ultimately your right, difficult impedance loads (not necessarily but jagged, irregualar dips and spikes) really do need SS.

Just a thought you might contact Kevin Hayes at VAC and get his thoughts. I have a Ren 70/70 and it has plenty of power for Dynaudio drivers although the crossovers are custom. I know the speakers efficency is in the high 90's. Kevin's amps have tremendous transformers and really deliver more then there stated output in my experience.

Roger