Well, if you've got a money back guarantee, then give it a listen. OTOH, depending upon what program you're using for playback, there might be software add-ons that'll give you the option of altering the sound. Also, there are some products like the Behringer Ultra-Q for example that you can insert between the amp and preamp (analog too) where you can dial in the amount of tube sound you want as well as some basic parametric equalization. The thing is, if you go with a 'box' then you're buying into what one person thinks is the way to alter the sound. Might be good for some stuff. But for all?
Well I don't have any measurement equipment, but I didn't get the impression that they were adding "stuff" back into the audible frequency band.
What peaked my interest more was the usb>I2S>DAC>anaolg out as a desirable interface for a 44.1kHz bit-perfect streaming music server. The DAC is available with/without the tube stage, and that really wasn't my main focus, the elimination of the usb to S/PDIF conversion was what caught my interest.
You know of other usb>I2S>DAC>anaolg out solutions ?
Plus I see a lot of acolades for NOS DACs in general, although I've never listened to one.