IMHO the answer depends, but I would not choose any of the amps on your list. That does not mean you should not.
My feelings:
LB30 2.N. I think it's way too expensive and I would not benefit from the added features that contribute to the high cost. Additionally, the speaker emulation is, or will be obsolete and superseded shortly. If I was into IR loaders I would go with a separate unit such as the new Two Notes OPUS which recently replaced the CAB M+. Then when it becomes obsolete/unsupported I can simply throw it away rather than having it cluttering up a lovely amp.
CTM15. There is a new CTM15 model. The previous version was reported to sound more like a guitar amp. Not enough headroom or lows for bass. Also if you are a tube purist, the front end of the previous version was solid state. No idea how the new model compares.
PF20T. I chose the PF50T because it's a bit more versatile and flexible. The PF50T has enough gain to push into drive. Many users report the PF20T remains clean with Gain and Volume maxed. Pretty sure you can overcome this by feeding the PF20T a hotter signal. Also the PF50T is not what I would consider too powerful. I think my LB30s have slightly more clean headroom and the Little Stubby seems to have about the same amount.
I went on a low wattage tube amp bender over the last couple of years and procured the following: Ashdown LB30 Mk1, Ashdown LB30 Mk2, Ashdown CTM 30 Little Stubby, Ampeg PF50T. Each of these amps sounds great and has it's own personality. AFAIK the PF50T is a current model but the Ashdowns have all been discontinued...they are not hard to find used.
I generally don't practice anymore. Instead, I tend to noodle around on bass while I watch TV. When I am using one of my regular bass amps for this, I usually play slightly louder than any of my low wattage tubesters can go. So I don't have a particular use for low wattage tube amps, but it's been fun exploring and discussing their capabilities.
Based on your stated criteria I would say the LB30 Mk2 may be the best choice for you. It has a pretty strong baked in smiley-face tone contour. The lows are big and fat, but not particularly loose or sloppy. Lot's of air and glass up top. I think of it as similar to 80s HiFi (ala SWR). I think the Mk2 is less tonally flexible and also it excels more at clean than drive.
The LB30 Mk1 and Little Stubby have stronger mids and leaner lows. These are better at OD than the LB30 Mk2 IMHO. To my ears the LB30 Mk1 OD sounds more vintage, like late 60s to early 70s. The Little Stubby OD seems more modern.
The PF50T is all Ampeg. I think the clean sound is very similar to my vintage SVTs, but the PF50T seems a bit sweeter and more HiFi. I think SVTs are a bit more grindy. The PF50T drive is a bit different than an SVT. To my ears, the PF50T OD does not sound particularly good when playing solo, but it works well in the mix.
I also have a vintage Fender Princeton and a vintage Vox AC30 with top boost. Both sound great with bass.
