CAT JL-3 Versus VAC phi 300.1 monos?

MBL101eMKII or Avalon Isis
Two world-class amps that can drive most any speaker beautifully - among the best you can buy when you need a lot of tube power (more than 60 watts).
I don't know much about the CAT amp, (other than Ken Stevens is a great designer, and his equipment sounds incredible!), but I have auditioned the VAC Phi 300, (configured as a stereo amp), at a friends house. It was a great amp, with incredible treble response, (the best I've ever heard actually), and a lovely mid-range, especially for vocals. (The bass was very good, especially so for a tube amp, although we both felt that the Lamm M2.2 (220 wpc hybrid) monoblocks were better.) I can highly recommend it. And running them as monoblocks will help the bass response, from what I have read.

One thing I will bring up, is that the VAC amp does run fairly hot. And running two of them will certainly raise the temperature in the room several degrees unless you have air conditioning. (Other than this slight annoyance, these are great amps, and certainly are reference quality.)

Good Luck in making a difficult choice!
The CAT will make the VAC seem like an air conditioner:)
OP, have you heard either?
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.
IMHO, CAT stands above all in the amp dept, preamps are great but not the best...German press praised Cat amps 10 years in a row as best amp in the world!
These guys have MBl among other German brands in their back yard!
Soundwise, I don't agree with your suggestion that CAT stands above all in the amp dept. Nor do I agree that there is anything that is the "BEST". That statement ALONE is laughable and only individules with little experience fall for such.

I actually own MBL 101E's and also owned a pair of CAT JL3 Sig. MK2's.

I currently own MBL speakers and paired up with the VAC 450 Statement mono blocks and are by far my favorite.
Choose your speakers first, then the amps. MBL and Avalon are very different from each other. No sense in putting the cart before the horse.