Ok, a few things here.
I’ll start of with your amplifier.
Usually those 50W per channel specs are lower when all speakers are playing at the same time. Secondly, when playing at half volume it should actually be a lot less than half wattage since dB does not scale the same way as wattage. If you can give me the -dB scale the AVR gives you I should be able to estimate the power to each channel for you at that volume.
So, the other point is port velocity, I’d recommend having a read through this: http://www.eminencedesigner.com/EDTopic14.html
It mentions a given velocity. That should be an upper maximum really, if you want to minimise distortion, aim for 17m/s or below.
To do this it’s not really about enclosure size. A larger enclosure typically makes the port system more efficient. What you’ll want to do to is increase the port area (diameter if circular). This also results in a longer port for a given tuning frequency. Annoyingly winISD doesn’t take into account the volume a port and driver takes up when considering the enclosure volume, so be sure to add that on afterwards when building your enclosure, else your results may not match winISD as closely.
If you don’t want to make your enclosure bigger that’s fine. Just know that there’s always going to be some limitations. If you limit enclosure size then either power handling or distortion/port noise is going to suffer. You have to weigh things up and decide what’s best for you.
Another thing is you should definitely make a prototype before making all the enclosures. You don’t want to make them all and then realise something was wrong in the simulation or that actually you need the port area and enclosure to be bigger.
Again, if there’s anything else, just reply and I’ll be more than happy to help 🙂.
I (and I’m sure many others) would love to see the progress of the build if you want to post about it on this forum as you do it? 😃