A deeper more holographic soundstage.

Dchang05,  isn't this how you describe your new system?

I purchased BHK preamp, M1200, and P15 together three weeks ago. The first week they sounded flat and dull, but now they are phenomenal.

So are they phenomenal or did you lose soundstage width and depth?

Your old tube amps had pretty high output impedance which could cause frequency anomalies and depending on your speakers you could have shelving of the highs with the 1200s. Odds are if you equalized frequency response your perception of soundstage would be more consistent.  Perhaps a better statement is that your amp, pre, DAC shouldn't impact sound stage (as intended) if they are neutral.

The deep wide former sound stage may have been artificial.

Twoleftears:  I agree with you. I have Ilumnia Magister speakers: Omniradial.1 meter from the wall  in 8-10°. Listeningposition in a “triangel”:only with that kind of speakers , you have a perfect 3-D sound. The chamber should also be “”echo-free” by using absorbent plates, if necessary. ( my system: CEC (cd-player), Metronome Technologie C6 (dac), Daniël Hertz  M6L ( preamp) and Wavac Md 805m ( monoblocks))

I read a lot of things written here as if they are gospel. I will give you my own experience.

 I have a holographic sense of space that reaches beyond my speakers and with excellent and focused imaging. I have substantial equipment between my speakers, and have no room treatment beyond an area rug and acoustic tiles in the ceiling. The room is built on a concrete pad, and thus the floating wood floor is firm. The speakers are ATC SCM35’s placed two feet from the wall and about 10 feet apart in a 20 by 20 foot room. I do not separate my cables, but I have a really excellent equipment rack of wood with steel points in steel cups separating each rack level. I do have a tube preamp, but I get the same sense of imaging depth with both a C-J tube amp and my tweaked-up, 45 year old Ampzilla, and whether I am streaming or playing vinyl.

So, while room treatment may help and equipment rack placement may help, you can get to this level of imaging without it. 

If you wear glasses while listening, take them off. Primarily tightens up center image focus (in my experience) which would secondarily maximize your perceived image depth (that already exists). YMMV. Other than that, looks like you have soup to nuts options presented here. Good luck. If any of you try this and you hear any improvement, imagine how bad the effect must have been in late 70's through the 80's when the style was lenses the size of dish antennae!

Erik_squires and millercarbon both mentioned elements that improved my soundstage depth the most. Treat the area in the dimension you are trying to improve. Front and back wall treatments will improve the depth of your sound stage. The engineer at GIK made this clear. Also, using a laser to get your speakers as symmetrical as possible side to side and vertically also is an important factor to consider.