Unfortunately, I did myself a service and a disservice with the EX600 when I bought the portable Zo subwoofer amp.
Prior to have the Zo, the EX600 - while no 1964-Q or FX700 level - bass was perfect for me with just the right deepness and quantity to go along with its outstanding quality. But the Zo has spoiled all of that for me now. When I listen to the EX600's bass now without the Zo, it sounds quite anemic to my ears (although not like the SE535, and with great extension and detail still). The Zo turns the EX600 into the type of universals I would have designed if I was responsible for the tuning at Sony. The Zo puts the Sony's bass level I would say just shy of 5 percent of the bass quantity level in the Quad (with the Zo at medium volume). It doesn't distort, but I don't like Zo with the EX600 at full red level. I do upper lower volume (greenish yellow) or medium (yellow, sometimes pushing to orange). So basically I don't enjoy listening to the EX600 that much without the Zo now. Like this morning I was going to commute with the EX600 using the Zo, but I the Zo had been running for over 10 hours straight and I discovered it drained this morning. Talking about disappointed. I put it on its charger and left it and the EX600 at home. I opted instead to take my Quads to work with me this morning (and I rarely like to take them out of the house). That Zo has become like audio crack to me...lol. Hello, my name is Eric and I'm a Zoholic!!
Anyway, tomorrow I think I'm going to give the Zo and Custom 3 a date.