To specifically answer the question, when dealing with 8 ohm you need to carefully plan out the location and wiring for each speaker and physically connect in series or parallel (as appropriate) to put no more than an 8ohm load on the amplifier. For example, two 8 ohm speakers in parallel equals 4 ohms - you'd need to wire those two speakers together in SERIES with another two speakers wired in parallel to balance back to 8 ohms.
THIS is why 70 volt exists! So that you can add a speaker or two or three without carefully tracking the wiring and keeping it all in balance.
A simpler answer may be to rewire just as it is now. If it used to work, it should continue to work.
Mike
