Advise on book shelf speakers less then $750


I am looking for advice to help me upgrade my existing Linaeum LFX speakers monitors.  I am looking for used speakers with a price around $750 or less.  That have open, detailed with very good imaging and that are small monitors.  The living/dining room I have the speakers in is approximately 13 feet wide by 34 feet deep with 8 foot ceiling.  I normally listen to music at a low volume of around 50 to 65 dBs measured at my couch 8 feet from the speakers.  I mainly listen to jazz, blues, and some rock.  The system consist of a Threshold400a amp, a Threshold SL-10 preamp, Macintosh 7082 am/fm tuner, and a first  generation Sota turntable.  Before you start and say that I need to upgrade my components, have no interest in upgrading them.  And secondly by budget number is fixed and can not go up.  Because of WAF the speakers need to be book shelf size.  What I really want are LS3/5a made by Harbeth but they all seem to be above my budget.  Any thoughts?  
waterford
Polk LSiM 703. They definitely have their own flavor but they are a fun speaker that are a joy to listen to. The Threshold gear should pair well with their darker presentation.

I might be coming into a pair (as well as the 707) via trade-in soon if you are interested, but if you want new, Adorama has been known to sell these $1,500 MSRP speakers for $500.

The LSiM is not your run of the mill Polk product. I don’t think they ever competed at their respective retail price points but street prices make them absolute bargains.
The Wharfdale denton is a good budget BBC type monitor. They are on the mellow and sweet side of the menu. 
These:
https://www.usaudiomart.com/details/649465938-lsa1-standard-speakers/
I wouldn’t be the least bit surprised if they significantly outperform the Harbeths.

P3ESRs and LS3/5as in general are not rock speakers. Why folks keep assuming Harbeths are a great solution for any listening situation totally baffles (no pun intended) me. They are mediocre at best for rock music, and all but their 2 largest models have "plummy" bass (think cheap paper OEM automotive drivers). Epos, Tannoy, and ATC are far better options for getting the British midrange with some real punch and drive.
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Linn Kans or Tukans, used around 300-400. NHT 2.1 SuperOnes. Both musical, good for 8 feet away. Maybe some LSA .5s, also in your price range.
Thank you all for the ideas.  The Wharfdale and Linn do peak my interest.  I have heard NHT before and did not care for their sound.  A little to bright for my taste.  I don't know anything about the LSA, will need to do a little research on them.                                 
Elac Debut 6.2 @$250.00 after a price reduction since they have been on the scene a few years now plus the Adante came out. Then there is Elac UniFi UB5 ($500). The B 6.2 i can vouch for and actually own. For 250.00 you get a very decent sounding speaker. The Uni Fi UB 5 i have not heard but they have really great reviews. Somehow i think they would be satisfying especially if you have  100+watt amp... Not a big risk if in my opinion.   
The Uni Fi is excellent, but I’m not sure it is so excellent at 50-65db. That is whisper quiet. 
The Uni Fi is excellent, but I’m not sure it is so excellent at 50-65db. That is whisper quiet. 
I’d second the Wharfedale Denton 80th Anniversary recommendation. I had a pair for just under two months and listened to them about six hours each day. They had opened up very well and were sounding pretty great with jazz and acoustic music. Not amazing for rock, but pretty good if a bit shouty in the midrange for classic rock. But, at your listening volume even rock should sound pretty damn good through these. 
I came across a very nice deal on a pair of Harbeth Super HL5’s or I’d be listening to them right now. Music Direct....60 day approval period. Heck, ask for the pair Carter DeVries  returned! They’re already broken-in! ;-)
Found B&W 685 S2's for $400 not long ago but one can find new ones for $700 and to my ears are transparent w/good imaging and handle rock, jazz and guitars well also have adequate bass. They seem to sound good with whatever you hook them up with and are fairly tolerant of placement since they are front ported. They are 8 ohms w/87 db's and amp recomendations are 25-120 watts. Also very clear for their price range. Also good vocals.
Parts Connexion has several "new in box" speakers for high discount:

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Wharfedale REVA-2 $519
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Monitor Audio Silver 1 $479 black / $536 gloss white
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Focal Chorus 706 REFURBISHED $449
I am looking for used speakers with a price around $750 or less. That have open, detailed with very good imaging and that are small monitors.
This sounds like a descriptor for Silverline Minuet Supreme Plus speakers at $699/pr new.  I heard Alan Yun demo them at a NYC show with (literally) a cheap-ass garage sale DVD player and Monarchy electronics, and it was one of the most jaw-dropping experiences I've had in audio.  That modest little system faithfully and visually reproduced a 3D standup bass in a fairly large room in a way I'll never forget.  And they're cute, small, and attractive as hell in rosewood.  And that was the first version Minuets.  I'd think your Threshold stuff would make them absolutely sing.  What are you waiting for???

The others that pop to mind if you can find them used would be Nola Boxers, Joseph Audio RM7, Reference 3A DeCappos, or some ProAc or Totem monitors.   Best of luck in your search.
Surprised no one has mentioned the KEF LS50 yet.....you should likely be able to find a nice used pair in that price range.