going from tube preamp to solid state

There’s a little bit of an elephant in the room I believe. You are much more likely to be able to adjust your soundstage and imaging with different speakers than any kind of preamp changes. Possibly you just love your PBN’s so now you gotta find what works with them . I think if you changed speakers you might find that is the path to you desired sound

SPL Elector preamp is a superb solid state preamp.  The biggest soundstage I ever got with a preamp is with my Black Ice F360 hybrid preamp.  You can dial in some impressive space.  I would though ditch the stop tube and put in a Linlai 6SN7 as it is much better.

I admittedly did t read all the comments but if you're looking for soundstage, a lot of that will come from speaker placement in your room. For instance, if you have an amazing center image but not a lot of soundstage width or depth I'm gonna guess that your speakers are 1, too close together, or 2, toed in quite a bit. I was having soundstage depth issues and really spent a lot of time moving my heavy speakers around and found a setup that worked extremely well where I get both soundstage width and depth as well as great imaging. My speakers are pushed to where they're about 18 inches from the side walls and 10 feet apart. I have ZERO toe in. I've had that setup with 3 different preamps at this point and my personal experience is that the 2 6sn7 based preamps sounded better than the 12au7 mu follower design one. 

I guess what I'm saying is play with speaker placement. The equipment you have should be more than capable of producing a nice soundstage. 

innersound300

"speakers have been moved in every direction possible and the end result is always the same : center imagining and that is where it ends. "

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something is wrong with connections, externally or internally.

L/R MUST be working equally to create a center image which is entirely Phantom. 

It sounds like something is in MONO mode, OR something is wrong with your l/r connections. out of preamp to amp; speaker connections. OR, inside the preamp. OR inside your amp.

1. Double Check all your connections, find something wrong, fix it.

2. double check all tubes: in correct sockets? (I switched my 6sn7 with 6sl7 once)

3. Prove AMP is producing Stereo (omit preamp). Hook up a CD player (any line level device (not LP) directly to the amp . Nice imaging? IF so, speaker wires are ok and amp is ok.

4. Same CD, same track: thru the preamp to amp. Sound same as when CD player straight to amp? 

a. you have two sets of outputs on that preamp, have you tried them both?

b. 4 inputs on the preamp, try each input, any difference?

btw, what AMP?

My experience was that I got solid imaging and wide soundstage when I switched to Mapleshade Golden Helix speaker wire. And you can try that for $150. And if you don’t like it they have a return policy. They want you to be happy with purchase.
Latest preamp is Audio by Van Alstine Vision RB preamp. It is SS and beautiful sweet analog sounding to me.

happy hunting!