Radial Firefly?
 

I came across this tube DI and I’m kind’ve intrigued by it. However, I’ve heard very little about it, and haven’t found any info on this forum, which makes me wonder... Anybody have any experience with it?

While I don't have direct experience with this unit, I've recently been using a Palmer PDICTC. And though it was an impulse Black Friday purchase, it does sound really good. Thick and warm. Two completely clear audio terms ;) Still, the Palmer is more expensive, bigger, heavier, and a different feature set. Not to mention it appears to be discontinued.

You might try bass guitar forums. Seems that's the biggest market the Firefly.

I reviewed that one for Acoustic Guitar years ago when it came out, so I had it for a while. Very nice unit, a bit pricey. As I recall, it's more geared toward electric guitars (as Spook says, it should be good for bass), with it's low variable impedance control for fine-tuning magnetic pickups. You also get a bit of tube "warming" from it, which might be useful for acoustics.

I have one and I love it. It is pricy but it’s quality. The only thing I would change if I could would be the ability to use inputs A and B at the same time for a dual source blending type thing. It does make using two instruments easier. I’ve used it with great success switching between an Anthem equipped d35 and a piezo equipped mandolin. The feature set on this thing is very robust. Lots of routing options and the fx look is nice for instruments to share fx between the two channels. It is not a neutral preamp, by which I mean it definitely colors your sound, but in a really nice way. You do have to watch your input volume as it is easy to saturate the tube and get some distortion if you aren’t careful.

Volume issue
 

I just got my Radial firefly today. But when I connect my guitar into guitar Amp through AUX of Radial firefly with post setting, the volume seems reduce a lot. I need to increase both Trim A and level button more than half in order to have reasonable sound. Given that the gain and master level of my Trace Elliot also more than half. Is there any other button regarding to the volume issue?

Wonder how it would work with a K&K mini.

I seem to recall trying K&Ks with it, tho I don't have much recollection of how it worked. Just looking at specs, you have two choices, a fixed 4M input impedance, which would work fine, and a variable impedance that can be adjusted between 22K and 500K. That range is rather low, tho the top end of it (500K) would be mostly OK with the K&K, and for those who find the K&K has too much bass, you could lower that impedance however far you want to lose some bass. Down at 22K, a K&K would be pretty thin. (Of course you could do that same thing with a bass tone control)

It's pretty common for DI's to have lower output. With passive ones, it's inherent, the transformer guarantees that the lower impedance output also has lower output. The Firefly is active, so they add some make-up gain before the transformer, but since DIs are meant to drive low impedance mixer inputs, usually mic inputs, they probably deliberately designed it for an appropriate output level. The Active part on the Firefly is more about adding "tube sound" than high output.

If you're looking for a signal boost, there are preamps that have a "DI" function with lots of gain. Grace Felix, Baggs Venue, Pendulum SPS-1, etc have enough output to drive a line input easily.

I believe the variable drag is for magnetic pickups.

For sure. It will affect anything you plug into it, for better or worse. But it's clearly meant for fine-tuning interaction with magnetics.

How do you think it'd word with Taylor's ES2? Is that a magnetic pickup system?

Reference below..

The ES2 is an active system - it already has a preamp that buffers the raw pickup. The "drag" impedance adjustment will have little or no effect in this case. The idea of this feature is that you can adjust the tone of a passive pickup by changing the impedance. With a preamp between the raw pickup and the DI, that's not going to happen.

It's still a fine DI, you just won't benefit from the impedance loading/drag.

So, in other words, it really won't be able to be used in the way it is intended? Or at least one of it's most notable features won't make any difference?

I certainly wouldn’t call the variable impedance one of its most notable features, it’s nice if you are playing with a passive pickup, but I wouldn’t write this DI off simply because you don’t need the variable drag option.