Long and involved speaker question

[Excalibur]

OK, this could be a bit tricky, but please try to stay awake as long as possible. I possess three pairs of full range speakers, and three amps. Two pairs of speakers are old, and of Heinz 57 components. The third pair is newer, branded, and 'tho small does 75% of the work. One of the old pairs requires a tweeter upgrade, and will be just fine. The other old pair (and one amp) may be regarded as backup/spare/hire.
Now the questions: I know that to get more volume with 8ohm speakers, adding a matching pair in parallel drops impedance to 4ohms, allowing more power from the amp, and a second set of speakers pushing air about results in a satisfying increase in volume. Therefore buying another 8ohm set has obvious advantages.
However, now the bit I don't think I understand: If I were to add 8ohm subs, and feed the tops from the built in crossover, does the impedance drop to 4ohms or remain the same? I can see that the sound quality should increase this way, but I can't believe it's that easy for a volume increase. I think the impedance will remain at 8ohms, meaning the way to go,would be 4ohm subs, and two pairs of 8ohm tops!
I don't want to start bridging amps, buying new amps, or bi-amping ( cos I don't understand that at all!)

Thanks in advance for any (helpful?) replies.

[spin mobile disco]

No I think the impedence would be 4 ohms when you add 2 8 ohms together, most tweeters dont add much to impedence either way.

[Paul James Promotions]

No I think the impedence would be 4 ohms when you add 2 8 ohms together

Correct.

[spin mobile disco]

Lol thought so

always nice to get agreement though :)

[Excalibur]

No I think the impedence would be 4 ohms when you add 2 8 ohms together, most tweeters dont add much to impedence either way.

Indeed yes, but that is parallel. According to the manufacturers, whom I contacted, your impedance through crossovers is determined by your subs, so 4ohm subs feeding 4ohm tops apparently doesn't drop to two ohms.