So, I bought a Ditto looper a couple months ago and have had fun playing with it. I wanted something simple and cheap to get my feet wet with looping. It sounded like it had what I wanted, in terms of no frills but high quality for what it does.

However, I'm getting somewhat disappointed in it, and here's why: If I'm playing my guitar plugged through the Ditto, into my amp, there's "true bypass" and the tone is strong. Then I loop my playing and start the playback, and the tone is just not the same. It's like a pale shadow of the original tone. I'm not able to build layers of loops that I like when the looped tones are so weak.

The Ditto advertising says that tone preservation was their main concern, and they deliver "24 bit uncompressed high quality audio," which sounds like it should be good. But it's not.

I'm wondering if others have this issue with the Ditto, and if it's an issue with all loopers. I'd love to find a looper that truly recorded and played back my tone without this degradation.

Bernie Sanders for President!
I don't have a Ditto, but I do use the loop function on my Flashback Delay from time to time, which I think probably relies on the same algorithms and programming to perform loops.

Anyway, I agree, the tone is not the same. The tone is a bit muddier than the original track that you played. I've found that by turning up the "level" knob, the tone improves and sounds closer to where it is without the pedal. I really only use the loop function to hear what my layered guitar ideas sound like -- it's very useful for that. But I don't ever see myself using it in a live situation, at least without a great deal of practice first.

One other thing to keep in mind is that once you've recorded a loop, and especially in high-gain situations, your guitar is even more likely to cause feedback which can muddy-up your tone even further. You have to keep the strings muted with both hands until you're actually playing the next layered part.
Atmospheric dark metal w/ black and death metal influences:
(My Soundcloud page):

Pestilential Flood
I use a Ditto and, while it's clear that the loop is the not same quality as what is going directly out of the amp, to me, the loop tone is fairly good.

Could it be your power supply? Do you daisy chain? You tried the looper only (without any other pedals in the chain nor powered) ?
This may or may not be the issue, but I've noticed that a shoddy amp/speaker can make some bad sounds when multiple layers are put through it. Also, the ditto has a volume knob for the loop I think, you could bump that up some.
I don't think I've ever used a looper that sounds dead on. I've never had a problem with this and I'll usually use a different pup or pedal or something for overdubs anyways, I like a quick subtle tweak to make the different dubs contrast more.
Fender Mustang/Derfenstein DST> Boss Power Wah> Pedal Monsters Klone> Bogner Uberschall> Walrus Audio Janus> Randall RM20> Line 6 M9> Randall RM20
I have the one control knob for the Ditto set to "noon." It was my impression this controls how much of the output is the recorded loop versus the live playing. So I thought if i turn this knob higher (to get a better quality looped tone), then I will have trouble hearing my live playing / harmonizing, which kind of defeats the purpose. Perhaps I'm wrong on how that works?

It's not a power supply issue, the Ditto is my only pedal. I have it going into a Yamaha THR10, which is a small, kind of cheap amp. But the point is, I'm finding setting that I like when I'm playing through the bypass or recording, but then it turns weaker / limper / muddier on playback. So, granted the amp is not going to give me the same tone as a Mesa Boogie tube amp, I'm more concerned with the disparity between live and recorded tone.

Well, I'll try turning up the knob, see if that helps or (as I feared) it drowns out my layering.

Bernie Sanders for President!
You do realize that a looper works best when run into the cleanest possible amp. A powered monitor or PA being the most ideal. If used with a guitar amp it works best in the FX loop, or only if the amp is on the cleanest setting. If your amp is coloring the tone with distortion or even delay it's going to muddy up the output of the looper.

The best way to use a looper is to have all of your distortion or FX in front of the looper. Anything else is going to degrade the sound.