single sub vs. dual sub enclosure

Hi guys. I will start off by saying that this is my first post. I wanted to introduce myself formally but i didnt see a new member thread anywhere. If i over looked it, I greatly apologize. but a brief bio of myself as well as my question... I have been a big car audio fan for years. Im talking back when pull out radios were the thing... Anyway, I recently got on the other side of the counter and began working at a local stereo shop here in cincinnati. I have taken quite a while but i finally decided to build my first enclosure. I ended up building a vented box tuned to 35hz. It is 6 cubic ft. I am running two diamond audio D6 15's. I have a diamond audio D31000.1 amplifier on each sub. I think they sound and hit great! Especially for my first box(with minimal help). However a couple of the guys that i work with that have waaaaay more experience and skill than i do said that a box the size of mine would be better for a single 15 and that i need to build one bigger. I was so confused because It is what diamond says is the perfect amount of air space. Do you guys agree or disagree with them? Shoul I rebuild a box that big for just one sub? All replies would be greatly appriated. I am very excited about learning from anyone that is willing to spend time with me on any aspect of my new field. Even more, I hope that one day, I can contribute even more.