I'm far from being a pro here, but it's something I've been looking at recently. One thing you haven't stated is what configuration of amp you are talking about. Solid state or valve? Generally a solid state amp will be able to put out more power through a 4 ohm output than an 8 ohm and even less on a 16 ohm speaker eg, my early model Quilter Micro Block will give up to 33W @ 8 ohms and 45W @ 4 ohms. I had another hybrid valve solid state amp that was rated at 420 watts at 4 ohm, 300 watts at 8 ohms and 150 watts at 16 ohms. With a valve/tube amp matching the ohms is much more critical. What I've only recently found out is the speaker efficiency plays a very important part in perceived volume levels. A 3 db increase in sensitivity or efficiency will give double the perceived sound level. Efficiency is something I look at much more closely than I did until recently. Hope this is a start for some info for you.
An amp set for 4 ohms should be matched to a 4 ohm speaker. If it is plugged into an 8 ohm speaker it will work, but the power is half. Instead of 200 watts coming out of the speaker it will generate 100 watts. That is OK.
The problem is when a 4 ohm amp drives a 2 ohm speaker. Now the power output is doubled. The amp might be designed to deliver 200 watts but is now trying to deliver 400 watts. That's when parts burn up.
A guitar playing friend of mine who toured extensively in the '80s maintains that you get a warmer sound from tube amps using the lower impedance taps matched to the correct speaker impedance.
On my three tube PA heads I can't hear a difference in warmth from one 10" 8 ohm speaker on 8 ohm tap to two identical 8 ohm speakers in parallel on the 4 ohm tap. Two can push more air, but I don't hear more warmth. ----------
It's not quite the same when the output transformer is actually wound with separate taps, thus offers a choice of connection points for various speakers. Some of those old PA heads also offer the option of 100 and/or 70 volt "line" outputs, which are meant for connecting long runs of Daisy-chained speakers, as used in applications like schools and factories. In the case of an unit which is tapped for a choice of speaker impedances, the mismatch effects don't really apply, although there is still some impact on the amp. Rather more subtle though.
In the case of running a mismatch, it should be said that this also places strain on the circuit. Some amps can handle it, but it can also be quite destructive. While it may be OK y to Run a 4 on an amp designed for 8, running a 16 with an amp calling for a 4 could be damaging. For instance. With some amps you probably shouldn't mismatch at all.
I'm too far from familiarity with this topic to give details; I'd have to go look it up. I know that when I built my modified Deville, I took advice to use the 2ohm tap and wire the speakers parralel rather than use the standard 8 ohm configuration. This was not about power or volume though, it was about more subtle quality of sound from the amp.
I decided to email Skip Simmons about this. I asked: "...I have one of your MA17s and have a question. Assuming a correct match ohm-wise between the amp and speaker, is there any reason to select one ohm option over another? Especially, any difference in sound?
Skip says: "...there is no advantage. You can also safely have a 100% mismatch either up or down and some people DO prefer that sound."
Edit: I guess that opens another question: How would one characterize the sound difference in the up and down mismatch? Hmm....
Last Edited by TetonJohn on Oct 14, 2020 8:34 AM