I check to see if the amps power doubles going from 8 ohm to 4 ohm.
Some cheap ht rec's might only gain 20-30%.
Krell KST100...... 100w @ 8ohm , 200w @ 4ohm, 400w @ 2ohm
Parasound 275...... 75w @ 8ohm, 120w @ 4 ohm
Pioneer A35R...... 45W @ 8 ohm, 65W@ 4 ohm