The Spider IV 150 is a 2x75w including speakers 'combo' amp and the Spider IV HD150 is a 2x75w head. You will need two speaker cabinets or a suitable stereo speaker cabinet to use the Spider IV HD150. The DSP sections in both amps are the same.
You have posted this discussion in the POD HD support forum. You may get more responses if you post questions relating to the Spider IV series amps in the Spider IV amplifier support forum