Recommendation for Good Quality Desktop Speakers

Passive version of KEF LS50. Not sure if your amp can drive it tough. Used prices are coming down now likely due to the active version of the LS50 being available. I use it on my desktop with a $480 ebay Parasound A23 amp. My KEF's were $1100 used 3 years ago. You can get them much cheaper now. I do a lot of late night listening in my office while i work. They sound amazing nearfield.

I also want to get the active version of the LS50 for another room (desktop like setup there too).

If you have the money, these should be great
Kef LS50 Actives with inbuilt dac 
http://www.digitalaudioreview.net/2016/12/kef-ls50-wireless-review-dars-product-of-the-year-2016/

Cheers George

Genelec is worth checking out.

Ancient Audio Oslo. Not well-know, but outstanding.

I upgraded from the Audioengine 5+ to the Audioengine HD6, and I use the Audioengine S8 sub along with the Audioquest DragonFly Red.  I highly recommend you take a look at these. Be sure to read the specifications and advanced features the HD6 has.

The New Proac Tablette 10 is very tube friendly
  offers an ideal 10 ohm load, are sealed for close wall placement, and sound incredible with many decent amps.
 Best,
  JohnnyR
 Proac Dealer 
http://www.tonepublications.com/review/the-pro-ac-tablette-10/

Silverline Minuette Supreme are very compact and musical. The Kef LS50 is very detailed and forward...if you are like me, this would sound great at first, but eventually fatigue you.  

I've been using for several years the JohnBlue JB3 single driver speakers in a similar setup with a 10 watt El84 tube amp. Obviously, if deep bass extension is important to you this isn't the gear you'd want but for near-field listening it's very satisfying.

just very slightly larger, but my favorites have been Quad 12L2

Hi -- I am thrilled by the desktop speakers I have been using for about three years. They are Audience Clairaudient "The One" speakers, and are only about 7" high x 7" deep x 5.5" width. They are single drivers so throw a great soundstage at a short distance.

The are not incredibly efficient at 84 dB, but will be easily driven by your amp at late-night listening volumes.

Clairaudient also has upgraded versions with the V2 designation. And it makes the slightly larger 1+1 desktop speaker that is many an audiophile reviewers’ desktop/mini speaker reference. Both have won several awards. 

I find them truly small enough to be a desktop or bookshelf speaker with excellent dynamics and a non-fatiguing sound. Worth a no-risk audition.

I use the Silverline Minuet Supreme Plus on my desktop.  They work great.