Very different animals. At these price points, you really need to listen for yourself. I’ve heard both, and considered them to be worthy for audition by McIntosh aficionados. Obviously the 3500 is simpler. And the 901 requires speakers with dual inputs, so Wilsons would be off the table. But there are lots of other speaker options.
My McIntosh dealer had a McIntosh 75th anniversary event at a luxury Bangkok hotel last week featuring Sonus Faber AIDA SE speakers (only 12 pair worldwide). He had the big 2.25kw amplifiers driving them. I asked about using the 901 amps, which I had seen driving Sonus Faber Aida speakers on Youtube.
My dealer said they'd tried it but the tubes burned out quickly. He decided that the 901s cannot be used to drive big, demanding loads.
Meanwhile we did not think the 2.5 kw with all the power management devices that were installed along with the amps and the digital room correction devices they were using (also McIntosh products) produced a very memorable or "engaging" sound. The room was hugely problematic because of all the glass and reflective surfaces. Too bad because I would have liked to audition these special edition Aidas in better circumstances.
Very different animals. At these price points, you really need to listen for yourself. I’ve heard both, and considered them to be worthy for audition by McIntosh aficionados. Obviously the 3500 is simpler. And the 901 requires speakers with dual inputs, so Wilsons would be off the table. But there are lots of other speaker options.