Can you use high level inputs on your subs or switch to subs that have high level inputs (driven by the speaker binding posts)? Most tube preamplifiers have high output impedance by their nature. The last thing you want to do (in general) is to add something else for them to drive that will make that situation worse, especially a pair of long single ended cables going to subs.
If you have to use single ended cables for the subs and want a tube preamplifier, you could try one of the LTA Microzotl headamp/preamp models (very low output impedance and good transparency) or use a tube buffer like Modwright makes (also low output impedance) between your source and integrated amplifier. I do not know what the typical output impedance of Mapletree preamplifiers is.
