I bought a Pre90/Ext90 on Amir's recommendation a few months ago. I use it to "bypass" my AV Processor when I'm just listening to 2 channel recordings from SACD or vinyl; a suggesting that came from asking advice on this forum. To do this I have my monitors connected to the Pre90 XLR output. I then use the Pre90 primary XLR input to take the signal from my Front Left and Front Right outputs from my AVP. This way, on startup, the signal from the AVP flows through to the front speakers automatically, at the preset input volume without having to press any buttons or adjust volume. Very handy. I can then change inputs to either SACD or vinyl, both of which starting at the input volume used in the previous session. Again, very handy.
The first task was to set the primary XLR input to match the output of the AVP dircetly to the front speakers. I did this using a dB meter close to my monitors and found adding +5.5dB gain to the Pre90 allowed it to be "transparent" to the AV setup, with regard to volume.
I use the Ext90 for the inputs from my SACD player and my Phono Pre-Amp.
I run a completely balanced Phono signal path: Tonearm cable terminated in a mini 4pin XLR into my MX VINYL, output in XLR to the Ext90. This is a very low noise solution which, considering my MC cartridge is very low output, really helps clear up any residual background noise. The first thing I noticed using the Pre90 is that I did not need to un the Pre90 louder than -25dB to get this listening level I enjoy. At 0dB, the cumulative background noise from the turntable to the phono stage and pre-amp was very low and far beneath the surface noise of the quietest pressings I have. The Pre90 did reveal a very faint power supply hum, which was fixed by re-running the tonearm cable away from a nearby power supply.
As for the SACD input, this works equally as well. I use a Reavon 200 for 2-channel SACD playback. I can't vouch for its signal quality in any objective way, but subjectively this sounds great. Much better than running the 2-channel though my AVP anyway. Which has made we wonder about the advantages of investing in one of Toppings outboard DAC solutions for all my 2-channel digital sources.
My AVP still plays a big part in my listening though. I have a 7.6.2 setup equally for home theatre and multi-channel recordings. I have a nostalgic interest in quad recordings from the 70's & 80's extending though to enjoying some amazing purpose recorded Atmos releases from the past 5 year. My main concern is to be able to listen to music with as much authenticity as possible, which is why my main speakers are the ATC SCM100 ASL midfield monitors; I've seen these in so many recording studios I have visited. The LCR set I have were originally used in a 5.1 mastering suit at Abbey Road before being sold to a recording studio in Santa Cruz. I rescued them out of the basement where they had languished after the recording studio was closed down to be repurposed as a Yoga studio. It took a year to completely rebuild them. My tastes far exceed my budget so I have to be resourceful.
I've used these monitors with a variety of Pre-Amps, including the recommended ATC pre, which sounds great but costs a whopping $6K. It's also a bit dated, especially in its range of inputs. Again, subjectively, the Topping Pre90/Ext90 combo does as good a job at the ATC solution, at 10% of the cost.
I'm very happy with my decision and the sage advice offered by Amir. Reading ASR has recalibrated my attitude toward HiFi. So much of audiophilia is a massive plate of Woo-Woo seasoned with magical thinking. It's refreshing to be able to read reviews that lack the abstracted, highly subjective language you'll see in traditional audio equipment reviews. And it's not that I dislike reading a florid, subjective assessment of a piece of electronic equipment; it's just that I enjoy more the serious evaluation of audio equipment without lavish claims of superiority that simply cannot be quantified.