CAT JL-3 Versus VAC phi 300.1 monos?

First response was spot on, both world class amps and you would likely be delighted with either.

I have lived happily with a pair of 300.1's for a couple years now so I can share that in my system and to my ears, they are beautiful....

From hearing the CAT amps at 10 years of CES shows and owning the 300.1 also owned CAT REF Preamp, my feelings are the Vac amps are a little more musical not tuby while the cat amps to me sound a little cooler in nature. I feel the Cat amps sound little faster and more solid state in the bass, but this also gives it a little cooler sound in general. Both great amps that will also depend on speakers and other sources used.

Jwn's comments ring true. The CAT is great amp for all sorts of speakers, but it is the tube amp I would choose for speakers that generally need SS to sound their best, they have power current and can really control bass drivers for a tube amp and in absolute terms as well. I don't mean this as faint praise, it also sounds great with most anything you drive with it.

The VAC may be easier to maintain. When I had the 300.1, a power tube went out and amp was still able to play music despite increased background noise. Switched the tube and problem was gone. With the CAT, there is a chance you may need new resistors. The bass is better running 300.1 monos.

I also have a pair of VAC 300.1s and have had them for a little over a year now. They don't run as hot as I thought they would, and it is a very small price to pay for the great sound others have described above. I've heard CAT amps at shows and while they also put out great sound, there seemed to be considerably more heat generated than from my 300.1s at home.

I also agree that the bass gets considerably fuller, faster and more palpable with 2 300.1s as opposed to just 1.