I would be extremely reluctant to purchase an expensive used Madrigal product unless you have a solid preexisting relationship with them - there have been a lot of nightmare stories reported on this and other forums over the years about Harmon's service involving used Madrigal gear (it seems the problems started when the Madrigal facility in Connecticut was closed six or seven years ago).
That said, I tend to prefer top-shelf solid-state preamps over tubed ones because they layer space as well as the best tube pre's, but are quieter, the noise floor in amplification components being of paramount importance in the very high-resolution systems these pieces tend to be used in. While I understand that the Ref 3 is very quiet for a tubed unit, the 32 regenerates power and is dead quiet, something you will clearly hear and come to appreciate quickly.
I also run my non-tube amp components 24/7 and do not know whether the 6550 in the Ref 3's power supply would hold up in 24/7 use (very possibly, but I would need to know), and if it doesn't, whether it is absolutely safe at the time of failure, whether failure of that tube takes out a resistor requiring soldering, etc. I have no fear about the small signal tubes in that preamp, but output tubes are another matter and can arc and engage in other scary pyrotechnics when they die. Does anyone know what kind of voltage the 6550 sees in the Ref 3's power supply?
Finally, the vast majority of tube preamps (ones not using output transformers, which is 99% of them) have difficulty driving long interconnects to the power amp without causing bass rolloff, so if your power amp is more than, say, 3 meters away from your preamp, this could be an issue. Tube preamps also have much higher output impedances than solid-state preamps, causing mismatches with certain amplifiers that again result in bass rolloff. The ARC Ref 3 claims an output impedance of 600 Ohms, but as is usually the case, no frequency range is given with this spec - 600 Ohms is almost surely the nominal rating and it will be significantly higher at low frequencies. If the input impedance on your amp is above 50 kOhms, or if your amp is just a few meters from the preamp, then you're probably okay.
Here is one of many threads that addresses Harmon's service:
http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1281863487&read&keyw&zzmadrigal+used+service