My entire last album is basically smothered in guitar feedback through an amp sim, as are most of my live shows. It really is all about SPL. I have done it with my studio monitors (and god I hope nobody thought we were talking about computer monitors!), but they are often in an awkward position, and especially with smaller near fields (and extra especially when you've got the rest of the mix coming through also) it can be tough to get enough of the guitar coming through without fear of damage. Then again, my entire band used to rehearse through my Alesis M1s and we had no problem getting guitar (or bass) feedback.
For
Great Big Mine/Drink This I used two different ways to get the feedback.
1) The two "main guitars" were done by splitting the signal - one side into a cleanish sim (
V-Amp Brit Class A and something else) which got recorded, and one through a high gain (probably Mesa) sim which was routed through a spare set of Altec-Lansing 2.1 computer speakers which were just fucking cranked. The distorted bass's feedback was done the same way, but I recorded a more distorted tone, and actually may not have had to split it.
2) I had one of those little surface transducer jobbies that you can stick to a plastic cup and play your iPhone through. Stuck it on the headstock of the guitar, stuck the guitar in the stand, ran the guitar through an all-pass filter and a loud amp model and routed that output to the "Boombox". Automating the all-pass filter has much the same effect as waving the guitar around in front of the amp. I applied a random LFO to that, hit record, and walked away for a half hour. Came back, switched guitars...until I'd recorded every electric in the house feeding back randomly. This runs under the entire song, along with some organ drawbar fuckery. You don't actually have to listen to it to get feedback, but it's anything but silent! Some of the high strings would take off and be ringing through the whole house. At one point we were afraid it might wake the baby. It does actually work for fretted notes, too. You can play with the thing and it works a lot like a loud amp in the room. If, that is, you can stand to have a cable hanging off the wrong end of your guitar. I kinda can't, and haven't found a suitable solution.
Here's one that you'll love, I'm sure: All of the feedback in
Nothing came from splitting my signal to my Diseased Dire Rat and then to
my AC4. So then you ask why I didn't just stick a mic in front of the AC? Well, it didn't actually make the sound I wanted when it gave me the feedback I wanted. And that is part of the fun part. You can record the DI and track and run it through whatever you want afterwards. You can get real string feedback out of crystal clean amps without having to evacuate the neighborhood. It does usually want some amount of compression because some of the good stuff happens when one or two strings are just barely ringing out, but the possibilities really are endless.
There's also always the idea of the magnetic coil sustainer system like the Fernandez thing. I could almost get behind that, except that they all pretty much require that you only ever play through the bridge pickup. That helps me on one out of maybe twelve tracks.
Honestly, though, for actual feedback while you're playing, there really is nothing better than shear SPL. The ideal for amp sim work would be a PA speaker capable of getting at least as loud as the amp you're modeling. Yes, I'm well aware that this defeats several of the purposes of using an amp sim at all...