I'll be honest and tell ya I have no idea what f's and xmax stuff does, but if the deltas are not suited for bass why would avatar use them? I used to own an avatar 410 with the deltas and I thought it sounded way better than the carvin cab I had before, and after owning an 80's era ampeg cab, I have to say I liked the sound of the avatar cab better, and it definately could handle more power.
And as far as buying another cab instead of speakers, I highly doubt I could get a new cab with new speakers loaded cheaper than I could buy three new deltas and just install in my used cab which is in very good condition despite having a ghetto speaker configuration.
I'm not going to tour with this cab! I had to sell my last cab to help with christmas, hence the reason I couldn't pass up this $50 cab. Right now it works, and when I get an amp to power it it will do for what I want it to do and I will probably buy a speaker at a time in order to spread the costs over a few paydays.