Audio Research from VSi75 to LS28SE/VT80SE or Ref 6SE/Ref75SE?

I've recently acquired a used ARC VSi75 integrated amp, and like it very much. It works well in my small room, where I listen in nearfield to a pair of Neat Ekstra speakers.

My sources are a Nagra HD dac and Nagra VPS phono stage with a PTP Solid 9 turntable and Audio Origami arm. The Nagra Dac and phono stage are both single ended.

Having just bought the amp, naturally I am immediately thinking of upgrading, and the LS28 pre and VT80SE power amp are the obvious choices. Has anyone directly compared the VSi75 to the LS28 and VT80SE? I'm sure there is an improvement, but is it significant enough to justify double the cost? Or is the VSi75 sufficient for nearfield listening in a small room with a relatively efficient pair of speakers (88db and 8 ohm)?

And is the Ref75SE a better amp than the VT80SE? I understand that the Ref75SE has balanced topology while the VT80SE does not. As I mentioned, my Nagra Dac and phono stage are both single ended, so I do not require the balanced inputs. I definitely prefer the appearance of the VT80SE, and it seems to be a more recent design, although not part of the Reference series.

And the Ref6SE is undoubtedly a better preamp than the LS28SE but is it worth stretching the budget to  the 6SE with a  VT80SE?

Keep the VSi75. It is already a good match for those Neat speakers in your small room. Spending more will not yield an improvement in SQ, unless you are willing to buy a pair of more expensive speakers.
Personally I think it depends on if you just want to try something different and of course the Ref75SE/LS28 would be the way to go.  Separates almost always get you better sound quality if the budget allows. 
No question The Ref 6SE / Ref 75se. These are world class components. What you have is good… but these are in a whole different league. I have a ARC Ref 6SE… it is a stunning preamp.. before this I had a REF5se. The preamp is really critical in setting the overall sound of the system… I can’t recommend it too much. 

While I am running a Audio Research REF 160s, I know of a number of people that upgraded from a REF75se, but just can’t let them go… they are outstanding amps.