What's the best EL84 amp?

Good question.  Many believe the EL84/6BQ5/7189 to be the best sounding non SET tube.  Power is limited to 15-25wpc in most applications however and that can be a problem for some.

I do have experience with this tube and amplifiers using this tube.   I have a fully restored Eico HF81 which sounds divine driving Spendor 15ohm LS3/5a speakers.

I am intrigued by the Musical Reference RM-10-II and will investigate it.   

I would also look at LEBEN for a new or recent amplifier.  My opinion of LEBEN in general is that their equipment is like a tube amp from the golden era that is restored and brought up to date with modern higher quality components but keeping the classic design.

I would look at the following vintage amplifiers:

Fisher 20a and 30a monoblocks.  Fisher SA100 stereo amplifier.  All 3 are superlative.  Deep bass, liquid midrange, and clear extended treble.

Eico HF14 monoblocks and HF86 stereo amplifier.  The HF14 may be the real prize- as it uses the same amplifier circuit and output transformer as the very well regarded HF81, and EF86.   But with a separate power supply for each amp.  This will improve isolation and deep bass response.   Prices are starting to rise for the HF14, but a cheaper way in may be through the HF12, which is a mono version of the HF81, or an integratged version of the HF14.   A simple rebuild to bypass the preamp stage and you have an HF14 at a lower cost.

pdreher, you may want to try a SET amp with those Altecs. You'd be surprised at how much you can get from 9W with very good iron. So far, my new RM-10 is actually a bit bass shy compared to the 300B SET amp I have. I've ordered other EL84 tubes to see if that's why but it's noticeable vs the SET and an old Scott EL84 amp I have. Both are tube rectified with huge transformers. I wonder if that's the difference?

@dhcod I occasionally swap in an Elekit 8600R 300B SET amp.  While it does sound nice with the Altec's, the bass is not as well defined compared to the RM9 MK2.  I have Werner Jagusch (MassMutter) crossovers on order... so I'm waiting for those to come in before I decide to try another amp.  I'll likely keep the RM9 MK2, but maybe fiddle with finding a backup.

...with huge transformers. I wonder if that's the difference?

@dhcod Yes, sort of. After dealing with the large potted transformers on the RM-9 which accounted for quite a bit of it's weight, Roger wanted a tube amplifier with a small footprint that he could easily carry around. So for the RM-10 he made much smaller transformers and compensated for them with networks in the circuit. The amplifier is extremely flat from 30 Hz to 20k Hz, but things start easing up below 30 Hz, so that might be what you are hearing.

At the time of it's design 35 watts from a pair of EL-84s per channel was double what other designers at that time were getting out of two pairs of the tube. Putting 700 volts on the plates was unheard of, until Roger did it. This is why the amp punches well above it's weight and has a pretty nice set of balls.

It has lovely balls. It's winning me over.