Hi, I have a digital sub 150. it's the entry level 12 incher. I have had it for about 3 years and I am very impressed with it. Very flat respnse, does not roll of sharply in the low octaves, which is great considering the price I paid for it. I can just imagine what the 600 sounds like. If it's my 150 on steroids, you will be VERY happy!
The 600 is in fact the bigger brother of the 150 you have. It has got dual 12" drivers and is 600W. Based on your impressions of the 150 I am thinking the 600 might definately go on my list of subs to seriously check out.