Have you considered that the problem may be with the Lowell PD system and not the Uniden scanner? I'm too far from Lowell to hear it at home, but I hear Lexington, N. Attleboro, Waltham, and some other systems without problem. I also hear the Boston 700 Mhz Overlay system. That's all on a 436HP with a 15 year old outside antenna/cable combination.
Have you programmed any of the NH P25 frequencies into your scanner?