Relate sensitivity/impedance to speaker efficiency

Atmasphere: I use an ARC VS 115 tube amp in my rig. Its rated output is about 115 watts driven into an 8 ohm load. My speakers are nominally rated at 8 ohms, but as mentioned above, their impedance curve drops in the lower frequencies and their impedance is more capacitive in the low frequencies. The amp/speaker combo subjectively sounds ok to me and plays quite loud, but maybe I'm not listening critically enough.

You mention that "[o]utput transformers in tube amps tend to be more efficient when driving higher impedances. Certainly speaker cables become less critical as the impedance of the speaker is increased. The result of this is that (all other things being equal) as you raise the speaker impedance it will appear to become smoother and more detailed, regardless of the amplifier used."

My question is whether I should think about raising the impedance load in my speaker circuit, perhaps by trying "high(er) impedance" speaker cables (if such things exist). I think I'm using Kimber speaker cables -- don't remeber which type. I also run my speaker cables off the 8 ohm taps. Do I gain anything by trying the 4 ohm tap?

Oh . . . one more factor to consider. ARC is replacing the SED "Winged C" 6550C power tube with the new Tung Sol KT-120 tube. I understand that the KT-120 tube will increase the power output of the VS 115 by about 10%. I also understand that the new KT-120 tube sounds better than the SED 6550C tube. Will the use of the new KT-120 tube change the impedance/capacitance analysis in any way? That is will my speakers sound "smoother and more detailed"?

Thanks Al and Atmasphere: I experimented with the 4 Ohm taps. Although I can not offer a technical explanation, I can only say that my speakers sounded terrible across the board when played off the 4 Ohm taps: way less efficient, rolled off treble and muddy bass. By contrast, everything was great on the 8 Ohm taps. As I mentioned above, the speakers are nominally rated at 8 Ohms, so I guess that's the way the manufacturer intended them to be played.

I plan to switch out the 6550C power tubes for the KT 120 tubes in a month or two. There's an outfit that breaks the tubes in for 72 hours and matches the tubes using three criteria. The cost is less than half of what ARC charges. I may try them out. When I do, I'll report back. Hifigeek1 is an ARC buff, so he may be interested. Thanks again.