Tube Amp settings: Triode vs UL; 8 ohm vs. 4 ohm ?

There is no harm in trying either 4 or 8 ohm taps, and likewise either triode or ultralinear operation.  You are the only one who can say what sounds the best to your ears and in your particular setup.  Are you sure it is an EL 84 amp?  With that name I would guess that it is a KT88 amp.

If you select the 4 ohm tap, you will be selecting a higher turn ratio for the output transformer of the amp.  This should tighten up the bass and reduce frequency response variations caused by the amp’s interaction with the varying impedance of the speaker.  But, there is a cost to using this tap instead of the 8 ohm tap, which is the amount of power that can be delivered to the speaker is lower.  Generally speaking, the best tradeoff would be to use the 8 ohm tap for speakers rated at 8 ohms and the 4 ohm tap with 4 ohm speakers.  However, the ratings are somewhat loose, so it doesn’t hurt to try the ‘wrong” tap.

A lot of people prefer triode operation over ultralinear for sound quality (smoother, less brittle sound), but the cost is lower output.  I have only heard a few amps where such a switch can be made for a direct comparison and I don’t recall liking triode operation that much more than ultralinear.  Again, give both a try.

This amp runs KT88 as output tubes, EL84s as drivers and 6922s as input tubes.  I would guess that it would have enough grunt to operate in triode mode even if that means about half the output of ultralinear.  You could also consider operating in different modes for different music/listening conditions.  Personally, I don't like to fiddle, so I wouldn't do that, but it is an option.