I am a bass player and so I can't say too much about guitar sounds and how the Mod is working for guitar.
I've bought the Mod Duo because I am using Linux as a DAW since many years and was thrilled with the potentials of using my "DAW plugins" live on my bass. Another aspect would have been to use the Mod Duo to explore some new sounds and some new effects I've never really used on the bass.
As a multi effect the Mod Duo works fine. You are EXTREMELY flexibel and there are hardly any limits. There are many plugins with variable quality (some are great, others have bugs or are not what you've like to have in a great effect processor). Then there are the unstable plugins - in my experience the rough ones in there are really rough, but there are some diamonds in there and some of the stable pluings are equally bad to me. It's great for people who like to tinker around and try create new ways of using effects.
I am using a preamp / power amp combo and tried to use the Mod Duo as an alternative to my pedalboard and my preamp. The signal path would be bass guitar -> Mod Duo -> Power amp -> Bass speakers. This is working fine, but my preamp (Aguilar Tone Hammer) sounds better. I've tried some different amp models, but I haven't found a way for me that to sound as great as my pedalboard together with the Aguilar Tone Hammer.
If you are a person who wants something that works plug and play, then I'd say that the Mod Duo will not work for you right now. There are some pedalboards you can try out but there are not presets like in most guitar modeling amps. There are no presets for a Marshall Sound or a Mesa Boogie Rectifier sound. There is the guitarix amp and other modeling amps - but at least for me they were not as quite Plug and Play as with "classic" amp modeling pedals.
Some answers to your questions:
- the background noise is no problem for me. Maybe my bass sound is not as HiFi as the people who have problems with it. Connected with a computer there may be background noises, but standalone the Mod Duo works great for me
- There is a looper plugin but I think it's not good at all. I get constant clicks and pops with it. It seems to work for some people (there is a bass guy in this forum with great videos) but for me it doesn't work at all compared to my Boss RC-1
- The new solution for different states of a pedalboard are pedalboard presets - but in my test I had pops and clicks when I was changing between pedalboards. So this isn't working for me either right now. The older solution would've been to use switches on the pedalboard (then you would need to Marshall instances) - but then the CPU power can become a problem (and the pedalboards tend to get messy).
for me the Mod Duo is a project with an enourmous potential, but it comes down what you're searching for if it works for you.