If you're looking for the ultimate garden speaker system then you should consider adding a subwoofer for extra depth to your music!
OUTDOOR SUBWOOFER TYPES
You have a couple of options here including on ground and buried in ground subwoofers.
IN GROUND: These are big, but discreet and offer amazing bass! Best for larger gardens where sound quality is a premium.
ON GROUND: Much cheaper and less groundwork involved, simply connect to a suitable amplifier and you'll gain more depth to your music.
OUTDOOR SUBWOOFER BRANDS
The Polk Audio Atrium SUB100 is a great little passive subwoofer that can be added to most stereo amplifiers along with a pair of speakers. It works well with the Atrium SAT300 satellite speakers.
For larger systems we'd recommend the Monitor Audio CLG range which will require a separate amplifier to power them, they also work within a 70V speaker system using the Monitor Audio IA power amplifiers.