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

Your amp requires 1.4vrms to reach maximum power.  Your sources are all digital which means you are already above 1vrms into your amp.

Let me pose the the question of why do you need a preamp?  Adding amplification that will be deamplified via the volume control can only add distortion.   Have you considered using a DAC with a built in volume control or a passive volume control such as a transformer volume control

As someone who builds tube gear and owns many pieces from various manufacturers, a couple pieces of advice when buying tube gear.
1 Understand the tube being used.  Look at the tube data sheets.  Look for high transconductance, greater than 6500 ohms 
2 Build quality is paramount.  If AC heaters are not done right your signal to noise ratios will be poor
3 Older products may needs rebuilt.  Many of the large capacitors go bad after a 20 years
4 Stick with a 9 pin tube as they provide internal shielding.  Unless it has a good DC heater supply
5. Do a little research on what a SRPP, Mu Folllwer and White Cathode folllower is and what their pluses and minuses are
6 Finally the quality of parts is paramount.  Poor undersized caps can lead to poor frequency response.  Carbon resistors color sound

As to passive preamps a good transformer volume control costs but they are very transparent and have plenty of drive as the excess voltage is converted to current.  With a tube preamp your minimum gain will be 20 which is 20 more times than your amp needs.  Which means the rest of it will be burned across a carbon pot.  Unless a good stepped attenuator has been used

BTW my main two amps are a parallel SET 300b and a 845.  I also have a MC225, a VT100, Sophia Baby Amp, and a Unison Reaerch 845.  Just to say I am not against tubes.

Of the 9 pin tubes the 12au7 is going to give you the lowest gain which when setup properly will be a gain of 17 to 20.  This is where looking up the tube data sheets come in handy.

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf

A gain of 1 will be a cathode follower or amtube buffer.  Tube like the 300b, 845, and 2a3 will have a gain around 4

Mudiness in a system with decent components can easily be caused by the room.  The book How To Get Better Sound is an excellent resource. Running test tones through the system (30, 50, 100, 200, 500, 1k, 2k, 5k, 10k, 15k at a minimum) will help you map the room.   A poorly designed tube preamp may de-emphasize the low frequencies, due to the undersized coupling cap, thereby reducing the the amount of excess bass in the room.  If you have already mapped the room and added diffusers and absorbers where necessary you can ignore this.

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.