Simon.<br />Trial and error my friend. If you are genuinely trying to find the "sweet spot" or more accurately making the sofa/sitting position the "sweet spot" then you are going to have to start at the beginning.<br />I am not aware of the JBL speakers you mention but the simplest way is too set the Sony speaker size settings to small (which will introduce a cross-over filter set at around 80-90hz, everything below this being re-routed to the sub) this means for the time being you can set the sub's crossover somewhere pretty close to 80-90hz (or bypass if it has one) and there will be little overlap (if the JBL's are large and you choose thelarge setting on the Sony which may or may not give you better results then you are going to need to know the "real" low frequency response of the speakers and set the sub's cross-over setting to somewhere around the same. The Stryke basszone test cd vol.1 (www.stryke.com) which has individual test tones from 10hz-20khz (about £10 delivered) and an spl meter will afford you good accuracy in these matters.<br />Once the cross-over settings are done using a known quantity either music or film place the sub at various positions around the room (i have always found personally that distance from the listening position is essential and the general perceived wisdom is "behind/next the sofa" is probably not to clever) adjusting the phase and volume accordingly. Personally i find music the better test as you can alter the DD/DTS lfe levels on the amp rather than the sub.<br />With careful and patient experimentation you will find it possible to integrate the sub totally seemlessly giving you an etheral quality that only introduces itself when called for not colouring, slowing or thickening the sound up AT ALL. Compromise dictates that most likely that "sweet spot" will require the sub to be 3 feet off each wall in a corner (read middle of the room - Unhappy spouse etc)<br />As said above when this is achieved when switching to DD/DTS if you feel it's to loud or too quiet you can alter individually the DD/DTS lfe output via the Sony, normally these settings or based around the analogue sub output setting which is a constant so setting up by a cd or even t.v is the best way to start.<br />Hope this helps, should you require further info there are many who worship the false god that is subsonics (including myself) who will be only to willing to oblige.