Single ended = large images?

I guess I don't understand how images can be considered too large. Are you saying images were larger than normal size humans, anything up to human size would not be unnatural to my mind.

Perhaps images seem excessively large due to sound wave interactions with room boundaries muddying up images. I've found judicious room treatment with differing combinations of absorption and diffusion does wonders for imaging.

My room is 13' x 30' , length extends into open kitchen which makes actual L even greater. I run speakers on short wall which means they fire down L. I've used and continue to use 845 and 300B SET amps, along with DHT preamp, this with extremely modified Klipschorns. Also have two Rel subs (crossed over at 32hz) which integrate with Khorns magnificently. I have near human size images that remain distinct within a large sound stage. Images sound extremely  natural to me in that they don't have sharply defined outlines, rather they decay and interact with other images very much like unamplified performers on a live stage. I did purchase a Pass XP-22 pre last year, image outlines more sharply defined which I didn't appreciate, one of the reasons I sold it.

In any case with 89db efficiency speakers you'll likely need 845 amp to adequately run them, and then you have to think about impedance slope, steep slopes require a lot from amp.I used to run 90db Merlin VSM with 845 amps, no issues as they had easy impedance load.

Many years ago I ran Cary SLM-100 push pull monoblocks, Dennis liked those leaky Jensen coupling capacitors, very warm relaxed caps. I remember experimenting with various Mundorf and VH Audio caps. In any case I don't recall any imaging or sound stage issues with these.

I don't doubt you can alter imaging to some extent with the feedback dial,  but this may open up a whole other can of worms. Tonality or tonal balance can be tricky when changing feedback.

To reiterate I don't see any alternative to 845 SET in your situation. You could try a variety of DHT tubes run in parallel or push pull mode for more power, another thread running at the moment discussing these. It seems you've had a good taste of SET, like it, so I'd suggest sticking with DHT tubes. I ran push pull amps with 6550, EL34, KT family tubes for years, I evolved such that for me DHT is the end of journey. Per usual amp/speaker integration key, those Cary's may have been just right for you with the proper match of speaker. As for specific 845 amp recommendations I've owned Art Audio Carissa Signatures, present is Coincident, very similar sound qualities. The exact 845 tube chosen has a relatively large bearing on sound as well, the Cary's may have had one of the warmer tube offerings.

I've never experienced instruments out of proportion to humans on any SET amp I've owned or heard at shows, or in other home systems. Now I do hear a certain sound stage difference between SET and push pull tubes or class a/b ss. SET's have this sense of bringing performers to the listening room, the others I hear myself more transported to the recording venue, no doubt this is a more up front presentation, may not be for everyone. Personally, I like both, wish I could afford and had the space for at least two reference system with these differing presentations.

@montaldo Completely slipped my mind, but in thinking about that much Rel sub power it came to me that one very important aspect of Rel subs is how they affect sound staging and imaging. I have only two of the T9x, adding these brought an easily noticeable expansion in sound stage and image size. I can only imagine how your stack of subs would far more greatly expand upon what I'm hearing. My room similar size to yours, I presume your stack with my 300B SET monoblocks would completely overwhelm my room.

With OP's six pack setup providing much of his sound stage and imaging needs he doesn't need a amplifier/speaker combo that provides a larger image, sound stage or up front presentation. Every SET I've owned, 845 and 300B partnered with physically large, efficient speakers provides exactly what he doesn't need. I'd suggest sticking with push pull tubes or solid state, more laid back perspective, sound staging and imaging more in line with his needs. For people who haven't experienced Rel subs you need to hear the special way they affect sound staging, integration such that they simply become extensions of your main speakers. The Rels are affecting the entire frequency spectrum in regard to sound staging and imaging. 

Both 300B and 845 SET I have sound stage and image differently from the many push pull tube amps or class A solid state amps I've run. I purposely made the move to SET because I desired fuller, greater density imaging, and the performers in the room perspective. Depending on perspectives some may call this bloat. Last year I purchased Pass XP 22 pre to compare to my Coincident Statement DHT pre, I didn't care for the way it more sharply defined image outlines, with Statement I get more decay or blending of separate images. While I wouldn't describe the Pass as having diminished image size, it did more precisely locate them in space. Perspective also changed in that performers became more distant to me. Been a while since I ran push pull tube, but if memory serves me right, the preamp experience replicates move from push pull to SET.