Palomino, when you say:
Quote:Not sure why hum on 8 ohm and no hum on 4 ohm.
Here's my take on a possible answer. The "Rachael" has a noise/hum rating of "<1.5mv" or about -72db according to Steve. The amp is designed to be most efficient with 8 ohm speakers from what I've read. From what Steve has told me, the "slight" hum you hear with your ear right next to the speaker driver is normal. It's what you should hear, at worst, so to speak, with speakers in the 94 or less db range. At best, you would hear nothing or perhaps a slight soft hiss. In all cases, from 3 or more feet away, you should hear virtually nothing in terms of noise/hum unless you have 103db, or greater, sensitivity speakers (again, according to what I've been told by Steve). This all assumes, that the hum is not being caused by other factors like: poor grounding, bad tubes, a problem within the amp like a bad solder joint or defective part. There could be a few other factors I'm not knowledgeable enough about to list.
From my own experience, two way speakers with crossovers are going to produce the inherent hum of the amp differently than high efficiency, single full range crossover-less speakers.
In the later speaker design (high efficiency, single full range crossover-less), given an accurate driver, there is nothing to reduce or modify the noise/hum coming from the amp before it gets to the speaker. In the former speaker design (2 way crossover) the crossover can soak up a good deal of the amp's noise (hum) making it very low to inaudible.
For example, on my 4 ohm MMG's, the Rachael produces virtually no hum (nearfield). However, the MMGs are known to be highly inefficient (about 83db) speakers which require a great deal of power. On the other hand, my vintage Allison Six's ( 86db, 2 way crossover @ 4 ohms) produce a hum similar to the hum I hear from my new Omega Super 3E (94db @ 8 ohms). On my Thiel 7.2 speakers which have a complex crossover network and are 86db 4ohm speakers, you can only hear a faint buzz from the Rachael with you ear right up to the woofer cone. So four different speakers with four different audible hum results.