I recently picked up a pair of these 500 series modules. So my experience so far is limited.
My first application for the Helios pair was mic'ing a pair of guitar amps for chunky rhythm guitars. I used 2 SM7B mics up close on a Mesa Solo Rec / sealed 4X12" Mesa Cab with Vintage 30s and an Orange AD30TC combo with 2X12" Vintage 30s. We needed to energize a big chorus on a pop-rock tune with the prototypical power chords. The amps were out in our live room, seperated from each other by a rolling Real Traps gobo. The guitar was a Les Paul (of course), but nothing fancy. We used a Little Labs STD guitar extender to allow the guitar player to sit with me in the Control Room. The Little Labs box has dual outputs, and we often use it to run two amps together.
The client started his palm-muted build up to the chorus, then let the sustained chord rip. He got really happy. He is a trained studio engineer in his own right, but remarked that he had never achieved a tone anything like that before. This setup just sounded great. The tone was solid, without ever sounding tubby. You could turn up these heavy guitar tracks a bunch before clouding up the mix.
Neve and API are different and both very useful, but the Helios is different from both. It does not seem to do the coloration thing that Neves do and APIs do a bit when running hot. The Helios' output is noticeably lower as well. If you like to print hot, you might want to use some make up gain after the Helios. On one occasion in recording this guitar amp combination, I used our VacRac TSL-4 for make up gain and a touch of limiting. It was nice, but I wouldn't hesitate to use an 1176, or any other decent compressor for make up.
Using the HF cut / boost at 10k can give you a range of tones from airy to dark. Despite the radical boosting you can do in the high end, you would have difficulty making it sound harsh. Rolling off some top end yields a smooth, dark tone that still sits well in most any mix. The mid range controls (peak/trough) also gave us a lot of frequency options. There is even a non-original 16k selection in the mid range EQ. I don't know how much I would use the 16k, but somewhere down the line, who knows. The mid range controls gave a variety of tones, none of which sounded bad. Some people gravitate toward certain frequencies, according to online posts. But I find most all the frequency selections useful.
I never found myself using this EQ correctively. It was always more about getting a sound I liked, and the Helios can make it hard to decide what you like best. Turning knobs just makes it different, not so much better or worse. I mean that in the most positive way.
It is a great alternative to Neve and API. We have a bunch of vintage 31102s and a couple of the newer 1073LBs. We also have a bunch of API 312/3124 pres and a few 550B EQs. API and Neve are different from each other in a complementary way, but the Helios is entirely different sounding than both. Closer overall to an API on the preamp side of things, it is still not the same. The low end is more subdued on the Helios pre.
I also find the HPFs at 40 and 80Hz make more of a difference than I would have thought. I would use the HPF on this unit less than I would on the 31102s, our Chandler LTD1, or our SSL console. The filters just do too much for how I have used it so far. almost like a gently sloped 80Hz and 125Hz instead of 40/80. But again, that is only with my limited experience recording electric and acoustic guitar. I haven't used them for anything else yet.
I have read that these EQs are great for mixing snare, guitars, and bass. I can see that. The low end EQ has been compared to Pultec EQs. I really do want to try the LF control when mixing bass guitar and kick.
Minor gripes would include:
1. The HPF @ 40 and 80 Hz makes more of a difference than I expected. Perhaps on other sources, I will find the HPF more useful.
2. Also, as noted in the other review, the upside-downness of British switches and lack of clear labelling on the 48v could be a bad deal for ribbon users, or anyone who unintentionally hot patches a line with phantom active.
3. Of the two units I received, one had an issue with condensers. When I would hook up a condenser or active ribbon, the Helios would not pass any mic signal, but would produce a low hum. This happened whether phantom was off or on. After reporting the issue to the dealer, I am confident they will provide a replacement, but it seems like they are reluctant to resolve this issue as quickly as I would like. I don't get a sense that these are poorly made at all. Some of my best handwired gear initially came to me with problems. But they were fixed quickly and have given me many trouble free years of operation. The real drag is that I now feel like I NEED to have both these units in good working order, not just the one, AND NOW. They are quickly becoming go to units, even though I have lots of good Neve, API, Chandler, A Designs, and other units around. So I am losing patience for a fully operational replacement to show up.
4. Price is a bit steep.
This is a very good mic pre and fantastic EQ in a single space, lunchbox form. Is there another 500 series unit that offers that much functionality? AML has the 1073ez-500 with Neve-ish pre and EQ, in two spaces, at a good price. AML makes good stuff - definitely worth a look. But the Helios is different from so many API-ish and Neve-ish units, and is more rare, at least for now. I have two, but want more. At 1550 USD a piece, I think it is a worthwhile investment to have a few.