I know guitar feedback naturally comes with high volume while playing in front of speakers, but there are some tips to increase it, such as compression, overdrive, etc. I'd like to know if there is a particular eq setting to improve (positive) feedback as well.
The way that I get lots of feedback for infinite sustain is to use a compressor with the sustain set as high as it will go. That usually does the trick, but you can also use a bit of gain on the overdrive pedal to boost the effect. If you put a wah pedal between the overdrive and the guitar, you can also tune the guitar signal for maximum sustain for your setup. These techniques can work with even modest volume levels.
I'm not sure what you want the feedback for. If it's simply for long or infinite sustains, you can also use a granular synthesizer or "freeze" pedal like the Electro Harmonix Super Ego pedal.
For feedback (positive feedback) you need plenty of gain. These sorts of experiments tend to be very setup-dependent, but when I did this I used a regular Ibanez solid body guitar, a Boss CS-2 compressor on full gain, a simple DIY fuzz box also on full, and my simple guitar amplifier (10 inch speaker) on very low volume. With this I got plenty of feedback in my bedroom, at normal bedroom levels, by sitting close to the speaker. (The volume was not high enough to damage anyone's hearing at that range either.)
So I would suggest the main thing to do is experiment with lots of gain, and modest volume.