If you're using the same interface and monitors while listening to others' clips (of people who use amp sims on their computers) and you like their sound more than yours, then I doubt the sound of the monitors or the D/A converters is a problem.
It's possible the input of your audio device is affecting the sound in a way you don't like. I have a Steinberg UR22 and I notice when I try amp sims on my PC they get "loose" sounding, where they didn't when I used to own an EMU 1212M. I know the UR22 has a stronger low end response on the input when I use mics (and I always have to be careful with my mics and/or run EQs to compensate), so it's likely that same extra low end response on the input is there when using the input for its DI (for guitar). If you have too much low end coming from your guitar, it'll make your amp (or amp sim) sound loose.
That being said, it could be your guitar. There's no guarantee that any model of guitar (made of wood, anyway) is going to sound the way you expect. Mahogany doesn't always sound a certain way, alder doesn't always sound a certain way, etc. It's about each piece of wood and how it's treated (if it's dried properly etc.) If you've already compared your guitars to a number of others and know that your guitars aren't too bassy/dark sounding, then I guess you can move on.
Try using a low shelf EQ before your amp sim. Roll off the bass before 100Hz. Maybe try 60Hz as the roll-off point, then gradually move it up. Guitars generally don't need bass in that range, and if there's any there it could be your issue or part of it.
Then there's the thought of: you're used to hearing others' amp sim recordings in a different context for the most part, in a mix. It's probably rare that you listen to individual tracks (mono, no effects, no doubling) of guitar. Maybe your current sound would work fine in a mix, but you're not happy with the sound on its own. That would make sense. Most of us don't like the sound of a single recorded track of electric guitar out of a mix context, and we generally all like the sound of our amps in the room (which is quite different).
You can also try different cab sims, I guess. And/or using different EQ for the cab sims as needed. Maybe you need to roll off some lows on the cab sims? If your cab sim plugin (IR loader, etc.) doesn't have a feature to roll off the lows, add an EQ there and try that.
Good luck!
I usually get the sound I want very quickly with a mic, and low amp volume isn't something that ruins my recording process either. I like the idea of impulses (and amp sims) but usually don't use them since it ends up being more work for me.