After a little bit of quick googling, depending on the circuit second order filtering doesn't seem that hard or complex if you really wanted to do it yourself. To make a second order low cut/high pass, you just duplicate the low cut RC (resistor capacitor) circuit again, basically applying the same low cut twice, doubling the slope of the roll-off. Hence, second-order.
Now, that's still an additional two components which have to be ordered, and assembled, and take up space. I could see why many amp designers would just change a value to have the low cut start at a higher frequency instead of duplicating it to get a steeper cut. And consider that high gain amps might have a dozen of these around doubling them up adds up in terms of cost and complexity. and more complex filters could need even more added to get second order effects there.
So I agree with Cliff that it's more costly and adds complexity, so most amp designers considering their amps on production scales would avoid them. It's also a bit annoying to add them to existing amps, because there won't be physical space laid out for the extra components. But it's a pretty attainable goal if you know what you're doing, don't mind it looking a little janky when you point to point wire a bit of circuit above the circuit board, and want to mod some into an amp yourself.
That said, if you wanted to try and build a FAS Modern it would be a lot easier if you were Cliff and knew which extra filters were added where. And even then, building an amp on the level of a Mesa Boogie Recto is a pretty big undertaking even before you go customizing it.
So for the OP, your easiest bed is to find a Mesa Rectifier series amp that sounds close, and make use of some combination of pre (before the amp), in the loop, and post EQ to try and get close.