I consider the use of the SDR mod to tame the treble peak coloration, and its making the HD800 warmer like the 800S, something you didn't want to do. Also, if you are using the SDR to "tame" the treble peak the difference between the Mimby and Uber will be way less/insignificant compared to the effect of the SDR mod. I can understand trying to get "neutral" but what if the "neutral" sounds terrible to you? I would think one would try to find something that sounds good versus an arbitrary notion of "neutral".
I'm not trying to tell you one way or the other but to hopefully give you a little to think about what you are trying to achieve, I might be way off but most people I know want to find something that sounds good to them regardless of coloration. Another option would be to use Sonarworks to get a flat FR which I feel is easier and cheaper than going with various DACs and amps unless you are not using a computer as a source.
Think you are a little mixed up on your terminology. There is a glossary of terms on Head-Fi and it's worth checking out:
http://www.head-fi.org/a/describing-sound-a-glossary
"Coloured - Having timbres that are not true to life. Non flat response; peaks or dips."
Meaning that, buy reducing the 6Khz peak you are making the HD800's
less coloured, not more.
The HD800S I personally won't not describe as warmer than the 800's. The have the peak fixed and a bass boost at the cost of resolution.
"Warm - Good bass, adequate low frequencies, adequate fundamentals relative to harmonics. Not thin. Also excessive bass or mid bass. Also, pleasantly spacious, with adequate reverberation at low frequencies. Also see Rich, Round. Warm highs means sweet highs."
The HD800S is still thin with most gear and certainly does not have excessive bass, mid bass or sweet highs.
Also, your use of the term 'neutral' - what if 'neutral' sounds bad to you... Well, that's possible. But if you don't like neutral sound then you are looking for something coloured (neutral would be as 'colour free' as possible) which is totally fine - it is 100% a personal choice thing, there is no right or wrong here.
You said, "I would think one would try to find something that sounds good versus an arbitrary notion of "neutral"."
"Neutral" isn't really all that arbitrary, what "sounds good" is totally personal. Some people love the TH900's and feel they sound excellent - I hate them. To each their own.
According to a comparison between the uber and multibit there seems to be a perceived 'fatness' in sound with the multibit. Warmer, richer, fuller, more bodied, etc. However, I am definitely looking to retain the characteristics of the HD800 without warming the sound. I want it to be as neutral as possible -- although the perceived 'fatness' may contribute to neutrality, although I feel otherwise. This is why I purchased the Burson Soloist SL -- no coloration.
Basically, something that has the least amount of coloration is what I'm looking for. I want to hear the headphone, not the gear. This is why I considered the ODAC, but I wonder if the uber does the same thing. (both are at the same price point)
What I think people are trying to articulate with the multibit is that it is tonally richer and smoother which can make the HD800 a little easier to listen too. I know your original question was about the multibit vs non but I'm not sure I understand your path... It sounds like you have HD800's which you are happy with (I love 'em too!!) and a Soloist and you are looking for a DAC. My question is why? What is it you aren't getting, or is lacking, with you have? What are you hoping to achieve by upgrading?
I think people here are trying to give you guidance and help but are struggling a bit because we don't really know what you are after.
Most people with HD800's ask something like, "I need a DAC that makes the HD800's more musical/thick/warm/less harsh/and so on" - they give us direction. You are contrasting two DAC's asking about their characteristics but are giving us confusing answers.
"I want to hear the headphone, not the gear." Then sell your HD800's and buy LCD-X's. Revealing your upstream gear is what the HD800s are all about.
Forget about terms for a moment and listen, figure out what you are and aren't liking (this is hard, I still struggle with this sometimes). Listen to as much other gear as you can and try to find what
you love. Start with the end in mind - what does a perfect setup sound like to you. Then ask for gear advice.
Anyway, that's just how I see it all...