Are there any $1500 tube preamps that that can compare with Tubes4hifi's SP14?

Have you read the book How To Get Better Sound?

https://www.amazon.com/Get-Better-Sound-Jim-Smith/dp/0982080700

If you have standing waves in your room a tube preamp may have nullified some the affect with less bass.   The way a basic tube preamp works is:
Source select to volume control 
Some sort of resistor divider to cut down on the input signal if the gain is too much
A tube
An output coupling cap

The coupling cap in conjunction with the input impedance into your amplifier makes a high pass filter.
cuttof freq is 1/(6.28*resistance*capacitance)
If any of the caps are undersized you might get a -3dB point at 15 to 20 hertz.   This would mean less bass

Your room has the largest impact of any component in your system. A good system is a hotel room that has not been treated will sound bad not matter how much it cost.

My suggestion is to do what you can to make sure you understand why you are having the problem and the strengths and weaknesses of the component the new component in question.

I purchased diffuser plans on line and had a carpenter make them.  This help the mids to low highs.  Added bass traps in the corners and that brought my room to neutral.  When you talk in the listening room it sounds very different than any other room.  Less echo and clearer speech. 

"The leap between the Mod Squad and the Audible Illusions in sound quality was monumental.  It changed the experience entirely. I love what I'm hearing."

I thought you were using the AI.

"I thought that the preamp had such a huge influence  that it would be the piece to continue to build upon."

In my opinion, the preamp is the most important piece to get right, assuming you made no major mistakes with your other components. Every component contributes to imaging and spacial presentation, so when I was looking your system over, I picked the components that were most likely the weakest in this area.

" So, if I was to swap out the preamp I have or the amp that I have, I would think a 1500 preamp vs. a 1500 (I could go up to 2k on that because the Bryston I wouldn't keep) amp would get me the bigger reward in improved sound."

You're over simplifing the issue. The biggest 2 mistakes you can make are using cost as a measure of quality and going with tubes because you assume they're better. System matching is far more important than price or design. In my own system I use $2500 speakers and have a pair of $10,000 sitting in my closet collecting dust. Tubes guarantee nothing. My advice would to be listen to a component and judge it based on sound quality. If it happens to be tube, fine. But if a SS components sounds better, then that's the one you should buy.

Just a comment on the advice to use a DAC instead of a preamp. Didn't work for me. The sound with the preamp was far superior. (ModWright 9.0SE and PS Audio Directstream DAC)

I got one of Don's first preamps.  It was fantastic, bettering my HK, CJ, Modwright, Berning, and any preamp I had had.  His latest iteration of the preamp is significantly better than his prototype, and I have had all the upgrades.   There is magic in 6SN7 tubes, as Don will tell you, and in my long life as an audiophile, I am simply in awe of the performance of his amp.  His KT88-based amp will no doubt be every bit as fine.  Don is a prince of guy to deal with, too.  Mark

The Audible Illusions Modulus 3a as an upgrade to your own 2 might do the the trick.  I've heard it in a good system driving a tube amp (Musical Reference?) and the imaging was to die for.  Of course, the owner knew his stuff and had the system well set up.