Moving away from inefficient/low impedance speaker in order to move to a Tube Amp

We had some monitors like that here, they were a similar 87dB and with my 20W Raven Blackhawk had no problem driving them satisfyingly loud in my 17x24x9 room. The guy who brought them had also brought his 100+ watt SS amp. We were all surprised how those inefficient power hogs came alive with the Blackhawk.

It is good you are figuring out something I have used as a speaker buying guide all my life- avoid low sensitivity speakers, they limit you on amps. But unless your room is huge or you rock out a lot (not likely, Harbeth after all) then you will be fine with any tube amp 20W or more.

As a general rule of thumb all amplifiers regardless of their technology make higher distortion into lower impedances.

       Agreed!

Have not done it but done a lot of research investigating the possibilities. Class D amps have kept me from having to go there (tube amps) to-date.

Not in general interested in large bulky speakers required for best performance in general with low watt tube amps.

However, to test the waters on a limited budget, I would likely go with Klipsch Heritage line and pick a tube amp from there. Could be Heresy, Forte, or Cornwall depending on specific room and what would provide the most flexibility there. Would also consider Spatial Audio in the right room or Fritz speakers (easy load to drive) for something more compact. I would also probably at first be inclined to give it a go with my existing Ohm Walsh speakers, but would expect to need at least 20 watts or so or more.

With a limited budget, practically, it would be hard to talk myself out of trying this

Amazon.com: Willsenton R8 KT88/EL34 x4 Tube Integrated AMP Power Amplifier Headphone (Silver) : Electronics

as a first stab. Lots of good reviews, good build quality, flexible, value.

Beyond that, if needed, on a grander budget, amps I have heard that I would strongly consider come from companies like Audio Research, Rogue, Jolida/Black Ice, Conrad Johnson, Linear Tube Audio, Audio Note and I’m sure several others.

Harbeth while not efficient I believe have an easy impedance curve for tubes. I think you would be fine with 20 watts or so. 

I just picked up a Jolida 302crc with very upgraded preamp and power tubes and it sounds very powerful on my Focals. No lack of power whatsoever. I believe it's rated around 40 watts per channel. This is also my first EL34 tube amp and I was expecting weak bass. NOT the case at all. I would not call it tight exactly but big and not too flabby. On the other hand snare drums that used to be just a snare sound now have a certain tactile feel to them. It's been a while since I have had tubes and I'm diggin it. Long term tbd.   

why not try one of the better reviewed Chinese tube amps some of them are quite good and you can get 50-100wpc easily. Willsenton R8 for example is quite good for the money and very well built. made in the same factory as primaluna etc. 

another option is vintage or older tube amps as the tech has not really change much in the last few decades other than refinements. There's a ton of quality use tube amps that will give you more power to drive your speakers and give you more head room. 

some examples: current adds here. 

Line Magnetic 211IA | Tube | Detroit, Michigan 48214 | Audiogon 35wpc

Black Ice Audio F22 50wpc balanced integrated tube amp | Tube | Bluff City, Tennessee 37618 | Audiogon 60wpc

VAC Avatar Super Stereo Integrated Tube Amplifier; MM Phono; Remote (35148) | Tube | Erie, Colorado 80516 | Audiogon

VTL IT-85 | Tube | Houston, Texas 77069 | Audiogon