So I was lucky enough to play around with a QOL in my system and here are my subjective impressions:
The build quality looks very good - along the lines of, say, Pass. It's designed to be flexible - e.g. if you have an integrated amp, you could put this between all your sources and the pre, but I believe the best way to use it is inbetween the pre and the amps, which is what I did.
As mentioned in reviews, there is a bypass button which you can toggle via remote but with the QOL enaged the perceived volume is louder so it's hard to A/B without the usual volume bias.
Listening impressions: I'm going to approach this from the standpoint of what my (amateur audio engineer) brain tells me is being changed by the QOL. First of all, the bass is pretty much unchanged. If I remember correctly, it only affects 125hz upwards. Everything above that frequency is louder - which changes the overall tonal balance to some degree. I found this mostly for the better in my system - at least at low to moderate volumes. I have plenty of bass, so more mids and highs (at least the degree to which it's augmented) seems good. It feels like it makes copies of all the information and mixes it back in with the original signal resulting in a presence/volume boost. Almost akin to 'double tracking' vocals and instruments (common recording practice to make a more palpable image) however in this case its doing it to the entire mix, not just an instrument. That means the ambience (or plain old reverb) is getting a boost as well.
The main effect to my ears is a more 'wetter' presentation. It adds 'space', as if the image was being projected in multiple directions instead of one. I suspect this is what people refer to as 'more like live music'. It IS engaging - no doubt about it, but it does come at a price: not all recordings suit this effect. For example, one of my test tracks is a Carpenters tune (yes, I do admit to listening to the Carpenters) where Karen's voice has plenty of reverb. With the QOL her voice is swimming in it and suffers from some intelligibility loss. It may still appeal to some folks - not like it sounds bad in any way, but imo it changes the character of the mix. Some track that had reverb did sound good - a Patricia Barber tune that has finger snapping sounded excellent and Patricia's voice, which already has gobs of reverb still sounded good. It varies. Classical music sounded good, as did jazz and rock (although I would still say this depends on the material). What I did find that some busy songs with instrument solos resulted in the solos being more buried in the mix (as if the engineer added reverb to it) where I prefer the more dry presentation.
Keep in ming that my Wilson speakers are very 'dry' sounding, especially in the upper frequencies. The QOL was never edgey and sounded quite smooth, despite the highs being more pronounced. I don't think I'd like this device in a very 'live' room.
The bypass switch is mandatory imo - and I think also in the manufacturer's. They warn that when playing vinyl, if an LP is noisy one may want to disengage QOL because it will emphasize the surface noise. While I haven't tried vinyl yet, I believe this to be true because with QOL engaged I can hear tube rush from my preamp at my listening position (where normally I cannot). It's truly being 'loudened' (and of course the frequencies for tube rush is midrange).
Ultimately I found that more and more I preferred my system with the unit in bypass mode.
