Exactly how are you using this amp sim? I suspect many folks just drop it on, pull up a preset, and end up disappointed and disillusioned. What would you do if you plugged into somebody else's amp when they've got the knobs set the way they like them, but it doesn't sound good to you? You turn the knobs. Excuse me if this seems elementary or you've already tried that, but I seriously don't think many people really do.
One thing that you always need to adjust for is your input signal level. If you just plug into any given instrument input on any random interface your signal level could be almost anywhere, and if you adjust the gain from there to make your meters look like your favorite guy on YouTube does for a miced guitar amp all bets are off. The plugin quite definitely does have some sort of nominal level that it "expects", though it's probably not well specified. It's definitely not spelled out in any real practical terms of how a given level might relate to real world voltages. But, you know, single coil pickups are half as loud as humbuckers. If you've normalized your signal like a good little monkey, you won't hear that difference in the amp. It's easy enough to figure out kind of where your interface sits relative to the plugin and work out an appropriate amount of gain or attenuation for your own rig. The way I record my guitars, I know I need about 10db of gain to hit the amp the way I expect. PodFarm has an input level and many others do as well. I set that knob to +10 and don't mess with the input level at all otherwise unless I really want the sound of something like a boost pedal before the amp.
Then you set the other knobs on the amp the same way you would on a real amp. I don't mean like exactly replicate what you dialed in on your favorite space heater. I mean do what you would do with a real amp - turn the knobs til it sounds good.
Now up to this point, it's like your amp is off in some isolation booth and you're sitting in the control room. This actually was pretty standard procedure in certain genres during the '80s. If you want to be authentic... But if you want something closer to what you'd get standing in the same room, you need some serious SPL. A Plexi that's cranked up to sound good hurts. It shakes things off of shelves. More importantly, it shakes the guitar. You need that.