@fjn04 How big is the room in which this speaker is being used?
If you are planning a lower powered tube amp and need to fill a larger space, the Harbeths won’t do the job. They are 6 Ohms, and Sensitivity is rated 86dB, meaning their Efficiency is less: 85.5dB. That’s not quite criminally inefficient... This speaker might work fine for a bedroom system where high volume isn’t a requirement or in a smaller room, which are more common in Europe where this speaker was made.
I’d be looking at something like the ZU Audio loudspeakers, which are easy to drive and reasonably priced- also DeVore Fidelity, Audiokinesis, Spatial Audio (if you have the room they are nicely priced and good performers) or Pure Audio Project.
As a general rule of thumb all amplifiers regardless of their technology make higher distortion into lower impedances. If you want the system to sound relaxed and more realistic, for this simple reason lower impedance speakers should be avoided (high end is all about reducing colorations which distortion causes). Distortion can cause the system to be more strident- that alone is a good reason to seek higher impedance speakers!
Efficiency (1 watt/1 meter) is an easier spec to use than Sensitivity (2.83Volts/1 meter) if you are planning to use a tube amplifier. This is because tube amps do not double power as impedance is halved. So efficiency tells you sound pressure based on wattage; sensitivity tells you sound pressure based on voltage. So if you have a 90dB speaker (which might be a nice number) but its also 4 Ohms, 2.83 Volts into that load is 2 watts, not 1. That’s a 3dB difference, so that speaker would actually be only 87dB and you would need an amp with twice the power to make it play as loud as a speaker that was actually 90dB with 1 watt!
I bring this up because tube amplifier power is expensive- and it always has been, which is why back when tubes were the only game in town speakers tended to be a lot more efficient. You don’t want to spend the cash for a good 100 Watt amp and I don’t blame you. So the sort of speakers you want to be looking to will have efficiency ratings in the mid 90s- about 10dB more efficient than what you have now.
That will allow you to get the same volume with an amp with 1/10th the power. The speakers I mentioned do that, with no loss of resolution.
Getting good bass is harder as the efficiency of the speaker is increased- above 95dB it gets really expensive. You might want to look into a subwoofer system. The trick here is to keep the sub operating well below 80Hz- that way it will not attract attention to itself and you can get it to blend easily. But to get good bass at the listening chair you may well find that one sub has to be placed inconveniently. Two subs will help but the best way to do this is to run 4 subs. Since bass is entirely reverberant in most rooms below 80Hz (due to the length of the fundamental bass note; 80Hz is 14 feet) or so, they can be run with a mono signal. Audiokinesis makes a sub system called the Swarm which is reasonably priced and designed to be placed directly against the wall. You might contact Duke about them. This would allow the main speakers to be smaller.
Audiokinesis
