Just for what it is worth I think it would be better to build the active tone control as a pedal and go all tube on the Crate V5. If you got the V5 to have a tube amp I think you should go all tube because any op amp in the signal chain to me is going to add a bit of sterility to the tone. So if the active tone control was always going to be in the front of the signal chain anyway if you keep it separate you will have some flexibility. You could mount the tone control in the back of the cabinet and do a patch cord bit to use it or not use it. I don’t mean to be a purist about this but plugging straight into a tube is not the same as going through an op amp first. You have bandwidth limiting with the tube due to the miller capacitance. With an op amp after a couple of dozen transistors a jfet or two and a feedback loop you have an output signal. As far as bass response that is relative to a lot of factors. The primary inductance of the output transformer where in this case using a 16 ohm speaker with the stock output transformer would be best. The speaker used and the enclosure the speaker is mounted in. With bass being a low frequency signal having an open back cabinet can work against you due to the reflected sound wave from the back of the speaker. On the plus side at approx 100 Hz most speakers are at resonance making them very efficient in that limited frequency range. Another thing is if the amp is a current amplifier or a voltage amplifier. With a tube amp without feedback the amplifier is a current amplifier so at resonance and higher frequencies as the speaker impedance rises the power to the speaker will rise. With feedback or on a solid state amp where the amplifier is a voltage amplifier the power to the speaker will not rise at resonance or with the changing impedance of the speaker. Relative to the speaker load an its effect the first part of the article I posted here covers the impedance aspect http://www.wattkins.com/node/17999
Also this on the current amplifier versus voltage amplifier concept http://www.wattkins.com/node/16905