What improvements would I get with a "better" preamp?

@patrickalston - great list! I'd throw in Aric's Super 6SN7 (I own an upgraded one); a bit less than the Motherlode but all in one box if you don't have the room for two. No XLR. 

Aim for a smooth, liquid sound—don’t waste money on it if it doesn’t deliver that. If you only need volume control, consider a passive preamp with an autoformer-based volume control.

Quick AI results (I don’t pay for ChatGPT or other AI):
 

A better preamp can improve your sound in several ways, depending on what you’re upgrading from and what “better” means for your setup. Here’s what you typically gain:

1. Lower Noise

A higher‑quality preamp introduces less hiss and electrical noise. This is most noticeable when:

• Using low‑output microphones (ribbons, some dynamics)
• Recording quiet sources
• Applying lots of gain

2. Higher Headroom

A better preamp can handle louder peaks before distorting. This helps with:

• Drums, percussion, and other transient‑heavy sources
• Vocalists with big dynamic swings
• Cleaner recordings at higher levels

3. More Accurate (or More Characterful) Sound

Depending on the design, a better preamp can give you:

Clean preamps

• More transparency 
• Better transient response 
• More accurate frequency reproduction

Character preamps (transformer, tube, discrete)

• Harmonically rich saturation 
• Subtle color or warmth 
• Pleasant soft‑clipping behavior

4. Better Stereo Imaging

High‑quality circuitry can lead to:

• Tighter left/right matching
• More depth and dimension in stereo recordings

5. Improved Build Quality & Consistency

Better preamps often have:

• Higher‑quality components
• More reliable gain steps
• Consistent behavior across channels

6. Better Impedance Handling

A good preamp interacts with microphones in a way that:

• Preserves the mic’s intended frequency response
• Improves transient detail
• Avoids tonal dullness or unnatural peaks

7. Specialized Features

Higher‑end preamps may include:

• Variable impedance 
• High‑pass filters 
• Pad options 
• Phase flip 
• Output transformers 
• Saturation controls 
• High‑quality DI inputs for instruments

If you really like the sound of the 2ASE,  have you thought of checking out the 2BSE?   Or how about the Line 2CRM?
I currently have a PS Audio BHK Signature preamp. I switched the input tubes and have been really pleased.  FYI, they just stopped making the and they are currently selling at half price, and can be found for around 3K used.

On phenomenon about the use of AI instead of forums to obtain answers is that it diminishes the library of information AI pulls from to generate its answers.   When we ask AI how a speaker sounds or its characteristics, AI, which can't hear, pulls from forums like this to develop its answers.  While it's true that it can 'see' test charts, it does struggle translating a peak at 4 kHZ into how that sounds to us, particularly when paired with particular electronics that were not used in the test charts.