The gain stage differences between the amp are unmanageable. FULLERTON and LONDON measured a 9dB difference which is twice as loud. Their other amps are even louder! Unless you constantly want to change your gain settings switching between amps, this is unacceptable. I also use UA DREAM, LION and ANTI, Strymon IRIDIUM, IK multimedia ToneX and none show this behavior.

I wrote to Walrus and after two months writing them repeatedly, never got any indication that they will fix this.

I wanted to wait for a few weeks of use to submit this review. First a little background. I have owned a number of amp sim pedals, such as the Line 6 Helix, Strymon Iridium, Walrus ACS1 MKI, UA Lion, and UA Dream 65. I also have a Friedman IR-D, which is killer. Although it is not really an amp sim but acts like one.
Getting back to the ACS1 MKII, the Fender Deluxe Reverb, Marshall Bluesbreaker and Vox AC30 amp simulations are what I use when recording and are excellent. Great clean tones, especially on the Fender and Vox. Great feel also. The Marshall JTM 45 cleans are nice, but the more overdriven tones are especially good. Turning up the gain I can also get Plexi Marshall tones. The cabinet IR files that come with the pedal are very good. I did add a York Audio Marshall 4x12 IR, but that is personal preference. Note: The ACS1 MKII also comes with 3 high gain amp models. Peavey 5150, Orange Rockerverb and Mesa Dual Rectifier. Since I don't use those amp models, I can't really comment on them.
The built-in display is a really nice feature and makes the pedal very user friendly. The built in Reverb does sound very good. The tone controls all seem to work well. The gain control is powerful, and I find it can really work to your benefit if used judiciously.
Overall, I love this pedal because it is so musical. It is inspiring to play and sounds fantastic on my recordings. I gave it a 4 out 5 only because of the limited presets it can store. but to be honest, unless you are using it live, that isn't an issue. Walrus Audio did a nice job with this pedal, congratulations to them.

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Takes a while to tweak it to get somewhat amp like tones out of it. My big issue is that it doesn't take OD pedals at all. They just end up saturating the tone. A completely different pedalboard setup has to be used here compared to one used with a tube amp.

Having paused playing three years ago due to the birth of my son and in need of something that I could play through headphones (but still with great sound) I was originally looking at a different amp simulator and placed an order for it. As always Pat Walsh quickly hit me up to confirm that I placed the order (as my account has been hacked before) and to discuss it. Pat and I chatted for a bit about what I was looking for then Pat clued me into the Walrus ACS1 mk2, at a relatively insignificant price difference ($). Instead of the 3 amp IRs and 3 cab sims I was looking at with the slightly cheaper model the Walrus comes with 6 amp IRs and 12 cab sims!!! I received quickly bc as always Pat hooks you up. Played all weekend while my three yr old napped and I'm blown away. Stuck on the Marshall Bluesbreaker (London IR) and the Fender Deluxe Reverb (Fullerton IR), getting sweet on the Orange Rockerverb as well (Citrus IR). Am really enjoying this if I had to ask for one more thing it would be greater volume (but I admittedly have hearing loss so I always want this). Thanks Pat and thanks Sweetwater for another great product.

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The four main reasons I tried the ACS1 are:
1. it was midi controllable.
2. it was the size of a normal pedal and fit comfortably on my pedalboard.
3. It has a built in boost that functions like a boost hitting the preamp, giving more overdrive as well as volume. The boost is also adjustable and saved as part of the preset. This boost function allowed me to remove the boost pedal from my board giving even more space for fun pedals.
4. although I had never owned a Walrus pedal I knew they had a great reputation and they were not going to release a bunk product.

I have only used it for several gigs so far, and I am mostly using it as a midi controllable Swiss-army overdrive pedal, but I really like it. Saving updates to presets on the fly is very simple, like if you need to volume match to another preset or up the boost level for leads. Also the midi was simple to set up, I am using it with the Strymon Circuit midi hub (which is a miracle of a device, I highly recommend it for 1/4 TRS midi controllable pedals). Note: the Walrus needs an 1/8 TRS and a hub like the Circuit uses 1/4 TRS, so make sure you check your hub and order the correct cable, which will probably be a 1/4 to 1/8 TRS cable.

I just received the cables I need to set the ACS1 up to use the stereo output when I have the space, but I have not had time to experiment with that. All in all this pedal packs a lot of usable features into a tiny little package and the gamut of amps it replicates makes it extremely versatile.

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6 amps, classics and high gain! Tweekable noise gate on board reverb, onboard presets, and it's midi friendly! This pedal replaced my Iridium and my Heavy menace!