I have 1&2 as a stereo pair and the rest mono outs going to my Audient Evo 16 via a behringer patchbay. They are 50 squid on Amazon these days and I’ve found it reliable for my small setup.
Sadly the Audient seems to need a level boost at a certain threshold that the outputs of the Isla don’t always meet so I have to gain boost some channels to make them work initially. Which is an Audient problem.
Love Audients interfaces but their driver consistency hmm, thinking of selling and just getting an SSL 12, fed up of dodgy drivers , even on Mac , class compliant just works.
In which case I’d run the main outs and a couple of individuals selectable when I wanted insert hw effects before printing.
Did you Normalize any of your samples before outputing them through the Isla’s S2400 outputs. Normalizing adds as much level as possible in a digital process only taking a few seconds to Normalize each individual sample. Try this process. It adds a lot of output gain. I bet this will solve your problem. I also heard you mention a slight hum in your Audient. Is that your analog mixing desk. I’m unfamiliar with this brand? Anyway, to figure out where this hum in your signal chain is tricky to figure out. I beleve you have a ground loop. I had my Soundcraft Ghost giving me a slight hum. In order to figure out where it’s coming from it’s a good idea to slim down your signal chain. Another words… do you hear the hum coming from your studio speakers when you first turn on your Audient mixer? If so the first thing to check out is your power supply. Where is your Audient plugged into to give it power? If it’s going directly into a wall AC you need to chnge this around. Buy yourself a grounded and sheilded multi power adapter supply box. Then plug that into your wall then the Audient into the grounded and sheilded multi power supply/ conditioner. These are the first to try. Also plug your powered speaker AC plugs into this as well as your Isla S2400. Turn down your speakers to center detent as well. I notice my Yamaha HS 5 studio speakers expel alot of air when turned all the way up to 100%. After turning them down to center detent 50% volume the signal to noise ratio was incredible. No more hiss. I noticed when turning on my 27" iMac I was getting a hum into my speakers even after turning them down to 50% so I was able to target that my hum was definitely not my Speakers, and not my Soundcraft Ghost console or my S2400. The hum was gone after turning of my computer. I have alot of gear in my studio that’s not mentioned but the way I was able to fix the hum was to move the computers power plug into a different multi power AC power supply that was also plugged into a different wall outlet AC. The hum was gone. So before you get rid of any gear see what the actual culprit is that’s causing hum. Also notice where it’s AC is plugged into and try switching it to an alternative power supply. Also, I hope you are using TRS to TRS sheilded cables from your S2400 to your Audient. Tip/ Ring/Sleeve cables are grounded and sheilded from picking up interference from other audio/ studio/ computer gear. It’s important to use TRS to TRS instead of just TS to TS cables or snakes. They cost more money for a good reason. They are worth it and will give you peace of mind without worrying about running power cables near audio cables and criss crossing cables etc. They also sell passive boxes that don’t use any power to do the jub of removing buzz & hum from gear to box gear. The passive box is great for a ground loop. They sell these boxes at Sweetwater Sound. These passive boxes are labelled like the one I use for my power amp going to my large Urei playback speakers called, “BUZZ OFF”. I connected my Crown Power Amp to Buzz Off and the other stereo side back into my Output 2 from my Soundcraft Ghost. The buzz was gone as well. The box is cheap small enough to fit in the palm of your hand and no power supply at all. Just TRS to TRS cables or XLR to XLR cables, the box has both. I forgot the name of the company that makes it as I’m not in the studio right now but it’s also known as Buzz Off. I would check it out. They also make these direct boxes to get rid of Hum. They also ground and sheild your signal from an AC ground loop. You’ll figure it out. Not a big deal but an easy fix. I hope this helps you out my brother 
I’ll preface by saying I make music for fun, not professionally. I use my S2400 with a 12u desktop rack setup that consists of the following:
- Samson mpl 1204 12 input 2 stereo bus mixer
- Samson sm10 mixer
- Roland sde 1000
- Roland dep 5
- JD 990 (with vintage synth card)
- FMR RNLA compressor
- Strymon deco v2 pedal
The 8 outputs (and JD990) go into the mpl 1204 which has the SDE1000 and dep 5 on the aux sends. The two separate bus outputs from the mpl 1204 go into two separate channels on SM10. I have the RNLA and deco v2 on the two aux sends on the SM10 going back to two channels in the SM10 instead of the returns so I can either send the deco signal into the RNLA or vice versa. Most of the time I have drums on one bus which I use with the RNLA/deco and everything else on the other bus (synths, samples etc).
For the most part this setup allows me to route any S2400 output or JD990 output to any of the processors without having to unplug stuff. I’m thinking dsp card will allow me to streamline this a bit and get rid of at least one of the mixers, but I’m not quite sure how yet until I get it.
Audient Evo is an 8channel audio interface. I didn’t say I have hum, not sure where you got that from lol.
I do normalise my samples , but obviously I don’t want everything battering the output and try and leave some headroom, maybe I don’t need to ,but with no level monitor I’m not sure what my levels are at.
Fortunately the Evo has input level meters , and I’m going in at about -12db on each individual in.
The issue with the audient is with its auto gain feature , which is a pain in the ass unless you are recording live instruments, it’s a bit pointless for machine music.
Edit: while trying to load old firmware’s on the Evo 16, which it wouldn’t do because the files stay greyed out, I had one last go with the latest.
Before I had just done it over the network, but this time I did it from a local file , great it’s working as should again.
Thought it only fair to clarify that the Audient recent update is fine , must have just glitched over external network.