Loudspeakers can be a risky item to buy second hand, with their delicate drivers and tweeters, that rely upon precise movement to provide top quality sound reproduction. Many older drivers used paper cones and foam surrounds, which easily disintegrated, just from age and general usage. Later drivers use rubber surrounds, which although more reliable, can still degrade over time. Modern driver cones use stronger more robust materials, e.g. B&W use Kevlar.
Tweeters are very easily damaged from being over driven or more commonly pressed in by small children. The cross over circuit inside speakers is another crucial component, which invariable contains capacitors – on old speakers (>20-30 years) these will likely need replacing.
Ultimately one needs to inspect and listen to speakers in person (or buy from someone why trust heavily). Careful inspect the drivers and tweeters (if the grills are removable). Carefully depress in the main bass drivers to ensure that move in/out freely without resistance. Check the surrounds for any damage, degradation. Look for any water marks, in case they experience damp or a spill. Tweeters shouldn’t be touched, so merely inspect them visually for damage. A common trick for fixing dented tweeters is to use a hoover or some adhesive tape to carefully pull them out again, however this will leave tell tail creases. Unlike tweeters, the centre cone/dome on bass drivers, is usually a just dust cover and so denting it has little or no effect on sound.
The key decider is to listen to the speakers, with a piece of music you are very familiar with. You may notice too much treble or bass, which will in fact be a bass driver or tweeter not working. Put your ear to each speaker, driver/tweeter in turn, confirm they’re all working. Better still, burn a CD with a full frequency sweep, starting a around 10Hz and slowly increasing to 20kHz, playing this will often show up any buzzing, crackles and rattles, that maybe less obvious when playing a busy piece of music.
Speaker with damaged cones or tweeters, maybe still a good buy if cheap AND a suitable replacement cone/tweeter can still be sourced. Replacing drivers and tweeters is relatively easy, a few screws and then swapping table tags (though sometimes soldered on). Even classic speakers with disintegrated foam/rubber surround can be repaired, assuming they are a standard size and replacements can be sourced. Contact one of the companies below to discuss any speaker repairs:-