This would be the first time I've heard of any 402x mic described as "bright" - IMHO they are the least bright major manufacturer cardiod mic (that I've heard). DPAs have a flat frequency response, and that is one of their greatest strengths, albeit also something that some will like, others will not. To me, the choice of pre really comes down to whether you want to respect that really flat sound by going with something transparent, understanding you may want to fiddle with the 'musicality' in post, or whether you want to intentionally change the color of the sound with your preamp. At least with the Naiant boxes, there seems to be a belief that the transformers in them color up the sound in a pleasant way - though I think the new obsession on the part of a lot of us with "transformers" may really be overblown and really doesn't focus on the fact that not all transformers are created equal and the way the transformer interacts with the other components (sound-wise, not in pure electrical terms) may not always be the same. An EAA PSP2 has custom-made transformers that certainly are believed to contribute to making it an outstanding-sounding preamp. But just because a PSP2 has transformers, a MixPre has transformers and a Littlebox has transformers doesn't mean there is some "transformer sound" that they all have that other pres don't.
FWIW I think if what you are calling "brightness" is really a concern about harshness, you may want to try the transformer-enabled littlebox or tinybox over the transformerless one. Owning both, I do think the transformer versions sound a bit softer while the transformerless ones tend to be really crisp bordering on harsh. I found the Microtech Gefell M210 hypers paired with the littlebox to be a positively awful combination (not knocking either piece of gear, just my taste) - horrendously harsh sounding. It led me to dump the Gefells almost immediately. OTOH, there are some really nice recordings made with Gefells and some of the "warmer" preamps, Myco's Gefell>Aerco recordings I have heard being one example. The Gefells have a really hyped midrange to me, so something warm tones that down a bit.
With my 4021s I thought either littlebox sounded pretty good, but overall I'd say I preferred the transformer one. Though I preferred the sound of either of the Sound Devices boxes (MixPre or USBPre2) to either of them.