which $1k used floorstanders for 35wpc tubes?

JBL L100 might as well be as floor stander with minimal elevation if necessary at all.

Tekton lore is high efficiency and in that range.

Zu Audio used, would be a nice combo with your amp.

Omega loudspeakers

I would recommend the Focal 917 Profile. Very nice speaker that's easy to drive. 91db efficient.

If you can find used coincident conquest, triumph even partial eclipse. They were made for that. Also, Soliloquy used to be very tube friendly. There's a bunch on usaudiomart.

"After the very-heavy NHT speakers, I think I'm really sensitive to cabinet resonance (I returned a pair of Klipsches for this reason)

There is so much to choose from on this site, and $1,000 is A LOT to gamble with! Help!"

I know the NHT's fairly well. If you had a bigger amp, you would probably like them better. For what you are asking, the best speaker that I can think of is the Vandersteen Model 1. They're much easier to drive and I can't think of an area where the NHT's can beat them sonically. They may be able to play louder, but that's it. (Providing you have a more powerful amp). Overall, it will be a very big upgrade and you won't spend anywhere near $1000 for a good used pair.

Monitor Audio Silver Series. RX6's can be had for well under a thousand used. RX8's might suit your room and music tastes better.

((I think I'm really sensitive to cabinet resonance )))
The Vandersteen 1CI feature a sophisticated box less approach.
Their low diffraction multi enclosure driver mounting system should work for you, add to that phase and time aligned drivers and are an easy drive with low to Med power tube amps.
I use Quicksilver 25 WPC Mono Blocks with superb results Johnny Cash's voice sounds genuine,like it should, Beethoven flows with dimensionality,and takes string tone where box speakers don't go.
The speaker is well respected and owned by many top electronic manufactures.
I would read the 1CIs owners manual just to get an idea
of what their about.
JohnnyR
Vandersteen dealer.
http://www.tonepublications.com/review/vandersteen-1ci-loudspeakers/
http://vandersteen.com//media/files/Manuals/1cmanual.pdf

It would be less of a gamble if you actually told us the name and model of your "35 wpc tube amp".

Also, since your amp and speakers are so mismatched, how do you know that you really love your amp? The NHTs are not bringing out the best of your amp and your amp is not truly showing what the NHTs do best.

I think I'm really sensitive to cabinet resonance (I returned a pair of Klipsches for this reason)

Guys,Try the Box test to hear the cow Moo artifacts
at 20 seconds in as CI sings the words 515 is just a train on your speakers this is what he is talking about.

http://www.songlyrics.com/chris-isaak/515-lyrics/
Cheers JohnnyR

With the Klipsch speakers, you may have heard the horn, not the box. I just went from Joseph Audio Rm25xl, which disappear in a room, to the Volti Alura,and it is hard to make it disappear, without careful setup. The horn does seem to pinpoint itself.

Also, the recording itself is Very important. On say The Woods Brothers "Magpie and the Dandelion", the individual guitar and mandolin are recorded directly from the left and right speakers. Not much you can do. Never noticed on the Joseph's but annoying on the Volti's. On well recorded music the band/ orchestra are in an expansive soundstage, and
the speakers boxes are gone.

Apologies to the OP for the off topic comments, I know it doesn't help with your search for speakers, but it may help someone else in the same boat.

Avett Brothers not Woods Brothers.

Rega RS1 or RS3 are VERY natural and as such are a prime choice for classical music.
Also, as they have minimal crossovers they are very easy to drive.Only speakesr mentioned I would put in their class are the Focals .

Some Ref 3A floorstanders Veenas?

For quite a while I was extremely happy with Vandersteen 1C's driven by a Rogue Audio Cronus tube integrated. 55 watts as opposed to your 35 watts, but the Vandies will easily sound fantastic with 35 watts of quality tube power.

This might seem like an unlikely match, but I heard the original Primaluna Prologue (35 wpc) played with Dynaudio Audience 72's and they sounded terrific together. Source was vinyl. I also support Focal and Vandersteen suggestions.

Silverline Audio Prelude currently selling for $750.00 on Agon. Perfect for your 35 wpc amp.
91 dB
8Ohms
Recommended amplification 10w - 300w

You will be hard pressed to buy a better floor standing speaker for the asking price.

Totally agree with Schubert, cannot beat RS3 for classical music but you will need to spend $1,400.00 unless you buy used.

That said, a pair of RS5s are currently being sold for $950.00 on Agon.
89dB
6 Ohms
Recommended amplification 30w - 500w

Classical music can have a very wide dynamic range which will be very demanding on the amplifier. Having enough power will be determined by volume and speaker sensitivity. Many of these speaker recommendations have a sensitivity of 88db-91db, and the current speakers are 88db. If the current speakers are underpowered, a 1-2db increase will not make a significant difference, and a 3db increase will only be a slight difference. IMHO, you need to be looking at speakers with a sensitivity of 93db and above.

RS-5 would be ideal for OP's space and 950 is a good price.
35 tube watts will do the job nicely.
If they were black I'd buy them myself.