Gregadd,
Your question is rather open-ended as you simply say that you are putting speakers in parallel & speakers in series. You do not mention anything about the type of speaker being used.
As you might remember, speaker impedance is freq. dependent. What are called "nominal 8 ohms" speakers are 8 Ohms over a very small range.
Further, what are the resp. efficiencies of 2 speakers i.e. what is their dB SPL measured @ 1m for 1W input? Depending on this, the 2 speakers demands on the amplifier could be very different. The one that is more inefficient could suck up all the juice from the amp leaving the more efficient one clamoring for power. This'll probably show up as unequal SPL of music for a volume knob setting.
The speaker x-overs will play a dramatic role in how they load or do not load the amplifier output impedance. This will make the effective speaker load difficult to calculate @ the amp end.
3rdly, your speaker wire will also figure into this equation. Using a cable that is highly capacitative could send your amp into oscillations (unless the manuf. ensures stability under highly capacitative loads). Using a highly resistive cable will kill your amp's damping factor & wreck the bass. This'll show up as flubby bass & as well as inadequate bass. In adequate bass will cause you to crank up the volume *possibly* furthering amplifier thermal runaway.
I would suggest that you use 2 speakers are identical in efficiency, variation of load impedance & type of x-over used, if possible. If not possible then try to use speakers are nearly identical so that the amp can service both speakers just as well. If neither is possible then ensure that you have a honker of an amp that can put out gobs of current to drive virtually any speaker & be stable at the same time!
FWIW. IMHO.
