A solid state amp delivers the power equally across impedance variations. A tube amp will put out more power at varying higher impedances resulting in a uneven frequency response similar to if you used an equalizer to boost the mids and highs up or down.
With a tube amp, sensitivity or efficiency of the speakers isn't the only thing to consider. BTW- your ARC amp is very powerful!
I have 11 different tube amps ranging in power up to 80 watts with KT 88, 6L6, 6550, EL 34, 300B and EL 84 tubes. Even the 2.3 watt per channel EL 84 Decware amp drives all of my 87-88 dB speakers in my 14 x 22 ft room well.
Loudspeakers which have series crossovers typically have a gradually rising impedance curve and a straight phase curve. unlike most loudspeakers with conventional parallel crossovers. Series crossovers use far fewer and much small components that don't suck up a lot of power and good sound.
Google any loudspeaker and see if they have a Stereophile review. Go to the measurement page & check out the impedance & phase curves. They will look like mountain ranges with peaks & dips. No other speakers will have smooth curves unless they use series crossovers.
Attached below is a graph of one of my loudspeakers with series crossovers. Thanks, Fritz
