I am only 15 and this is my first analog synth. I have a roland PK-5 bass pedal board and use it while I play bass guitar. I was wandering how low I can play with this rig
Well, I am not familiar with the Roland of which you speak, but the Little Phatty will go low enough that your room will be shaking even though you aren’t hearing hardly anything at all. That is, IF you have a decent amp. What amp are you running? 
I am running through a Roland KC550. The KC550 has a 15" driver as well as a tweeter. http://www.musiciansfriend.com/product/Roland-KC550-180W-Keyboard-Amp?sku=480195
Ok, you should have no problem getting down to 15-20Hz. How is a fifteen year old able to afford all this stuff, anyway? 
I Have a job at a mall, and I don’t drive yet, so All $450 per month goes towords gear!
Thats awesome! I remember those days. Well, congratulations on your first rig!