Here is the text of my review on Thomann for the Katana 100 2x12 and afterwards the review of the associated GA-FC footswitch, which is definitely recommended to make the use of the amplifier easier.
Pros: Excellent sounds, flexibility, build quality and volume. 4 channels so easy to use with a looper using a different tone.
Cons: No footswitch or USB cable supplied, no tuner, and mono only. Connections apart from instrument in are on the rear.
Recommendation: buy one before Boss change their minds!
I have had this amplifier for 2 months now, and I am still as in love with it as I was when I first fired it up.
God's teeth is it loud - I also own a Fender Mustang 3 V2, which I very much like, also 100w. But heavens the volume from the Boss has to be experienced to be believed. I am running it on 0.5w in the house and that is plenty (standing too close is not to be advised!). The build quality appears to be excellent - it's definitely a premium product. I had to update the firmware before using the Boss Tone Studio - a bit of a complicated process but easily enough done following the instructions carefully.
I've played all my guitars through it now, but usually use my acoustic Yamaha APX700 and my Epiphone Prophesy GX, the former using the acoustic pre-amp setting and the latter all the others. It has a much fuller tonal range than the Mustang, which can sound a little thin at times. The acoustic pre-amp is one of the main reasons I chose to buy this amplifier and it does not disappoint. Despite the relative simplicity of the controls there is a huge variety of tones available, all of them frankly excellent. The praise heaped on this machine for the brown sound is not misplaced. it's staggeringly good - as are the others. As I have grown more used to the amplifier I find the dynamic response of the crunch sound in particular is especially good. It really is simply excellent. The complimentary software is a little unintuitive, but works well. I've created my sounds which are now stored in on my laptop as a set.
The clean pre-amp takes external effects brilliantly, my Zoom G3X sounds better than it has ever done before.
I am giving 5 stars everywhere, because it just sounds so good, despite the niggling negatives: the amp is mono, there is no foot switch supplied and there is no tuner. And it is a bit pricey compared to the 1x12.
I now have a GA-FC which makes using the amplifier far simpler and enjoyable. The channels switch with very little delay - instantly for all practical purposes. Just a wonderful and wondrous machine.
And for the GA_FC:
In short: solid metal case, very 'sticky' feet, Katana supplied labels fit perfectly. Makes using the amplifier so much easier, and frankly essential for performance.
Detail: Plusses: chunky with a single row of foot switches. Red LEDs indicate which channel or effect is currently selected. Controls channel selection (including the panel) and switches the three effects groups on or off individually (being simple switches you cannot adjust any effect other than tap tempo parameters). It is also possible to switch the effects loop in and out It stays in place on the floor quite tenaciously.
Negatives: Does exactly what it says on the tin, although what it says on the tin is quite comprehensive, there is still no tuner which means running two cables across the floor to your playing position. The supplied control cable is cheap shiny plastic with moulded plugs and only 3 metres, so if you are serious about live use you will have to buy a longer better quality TRS lead. I am dropping a star in each category, total because it is expensive for what it is, features because there is no tuner, and quality because of the short and cheap control cable. The bottom line is that you do really have to have one if you have a Katana 100 or 1002x12.