What tube amp for starting out?

Excellent suggestions from Marco. All bases are covered.

I will only add that the Infinity Primus 360 have an impedance dip to 4
ohms in the lower midrange and upper treble (per John Atkinson's measurements in Stereophile), so if you decide to keep them, you should look for an amplifier that can handle that load. At the very least, it should double it's wattage into 4 ohms from its 8 ohm rating, otherwise you will hear a recess in these regions. Frankly, this might explain what you're presently hearing with the Dynaco amp when you say the sound is "dead in the midrange".

Good research Grant. Yes, with a significant dip in the lower midrange you might need a bit more muscle than your Dynaco can muster. I would sooner try more compatible speakers rather than sell off the Dynaco gear. But I don't know what of your system you may be more attached to (nor why). On the other hand, for most of the music you like to listen to, I would have suggested you consider SS or Hybrid amplifier, rather than tubes. It may not have the same warmth and dimension that tubes have, but it will have dynamics, speed and impact that your Dynaco won't ever be able to deliver. This is what I was hinting at when I said your idea of "realism" may be different from other's. Depending on what you regard most highly in the way a system presents the music, you may want to pursue a different direction. In my experience, no single approach does it all. There are always some compromises, especially so at this level of components and investment, and even at higher levels. That's not to say you shouldn't be able to have a great sounding system with some of the gear you already have. Weigh your priorities, find the weak links, and proceed.

Hey Grant, you and I should approach NPR and start a new radio program Stereo-Talk. Do you have strong regional accent? I can put on my Brooklyn accent - I lost it at age 15, when we moved out of there, but I can put it on when I need to.

Marco

Marco, I can do a number of accents as required. I do a particularly good Eastern Indian, but I'm afraid the present PC culture will not allow it.

At the very least, it should double it's wattage into 4 ohms from its 8 ohm rating, otherwise you will hear a recess in these regions. Frankly, this might explain what you're presently hearing with the Dynaco amp when you say the sound is "dead in the midrange".

could the 4ohm output taps be used then?

4 ohm taps would be a requisite with the Infinity loudspeakers, but this
is not a guarantee that the Dynaco doubles power into 4 ohms. The
Dynaco can probably be adjusted so the taps are 4 ohms, if they don't
already allow for this.

Another option would be trying Speltz Zeroformers that double, triple or
quadruple impedance...effectively doubling the Infinity's 4 ohm
impedance to 8 ohms, but they also would double the 8 ohm impedance
to 16 ohms, so the curve wouldn't change even though the load would
become easier. I've never tried them myself, so I have no idea how they'd
work, but theroetically they are an option for this scenario.

The advantage of an amp that doubles its wattage into 4 ohms versus 8
ohms is that it flattens out the speaker's response curve. Therefore
certain frequencies aren't emphasized over others, as would be the case
with the Inifinity's dip in the lower midrange and upper treble
frequencies.

Personally, I'd rather have a good speaker/amplifier match without
having to add fixes in between.