Not that either way is a good idea as in parallel you will cut the impedance in 1/2, so 8 ohm speakers look like 4 ohm to the receiver/amp. If the amp is ok with 4ohm then parallel 8 ohm speakers would be ok but still far from ideal. Give that you said you got great high volume out of them parallel is the most likely since the speaker impedance was 1/2 of what the amp was driving to.
The best if you are going to do that is if your receiver has 2 sets of speaker connections (A/B), then hook one set to A and the other to B and set the speaker selector to "Both" or "A+B" however it is labled.
If you had them in series I would think you would get a big drop across the first speaker and the second one would sound real low.
If you can remember, if you had them in series, then you would have had one wire come out of the amp going to the first speaker, then a wire from the first speaker to the second speaker, and then the return wire from the second speaker back to the amp. If both speakers on a given channel l/r had a pair of wires running back to the amp then you had to be wired in parallel.
One last note, if you have 4 ohm speakers I would not do this at all, your new impedance will be 2 ohms and near short and may damage your receiver or amp.
Right.
I bought a set of speakers waaaaaaaay back when I was a young, young man - along with an amplifier.
Harman/Kardon AVR35RDS is the amp
Over the years I accumulated and tested a bit with this set-up. I found the sorround-sound to be unsatisfactory in the end, and ended up just connecting more speakers in stereo.
This worked really well, however, I cannot recall anymore whether I connected them in a serial or parallel fashion, or a combination.
We're talking two small speakers with a high and medi tone speakers, both infinity/JBL - and two massive JBL speakers with huge bass units. This gave me a lovely coverage in sound, surprisingly, and I was able to play at very high volume with it sounding great.
I am, however, at a loss to how exactly I connected them way back when, and I was looking for some input. I remember I used some of those.. I'm not sure what the term is in English, but small plastic things you can connect wires in to?
At any rate, hopefully someone can help me out, as I want to connect it up like they used to be. (Shame on me for forgetting something this simple, I know..)
Thanks in advance!
Another more interesting variation would be front l/r hooked to the plus/minus for l/r and have the rear l/r wired the other way, but still have left on the left side and righ on the right side. 
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