accastil , to put some of your concerns in perspective :- {1} Digital or Analog inputs - try both options and whichever you prefer the sound of is the better one to use .
{2} Marantz receiver - this is not likely to be your primary problem here , at least not yet , so keep it till you sort out the next point ->
{3} The M-S 502 speakers - what "noddinOff" posted on page 4 is worth trying , so carefully read and try all the options suggested in the hi-fi choice link , AND , there may be some more options as the Mordaunt- Short data {elsewhere} seems to state that the speakers have a switch to select between 2 cross-over points for the subwoofers . If it does then try setting it to the lower of the 2 frequencies and with the polarity switch set to in-phase first . Next try switching the polarity to opposite phase , and listen if better or worse . The higher cross-over frequency option will probably worsen the phase problem that hi-fi choice reviewer mentioned . If there is an Equalizer for the sub-woofer , then try changing it also , but really these speakers were designed to work with an AV processor - as the M-S guy replied in the hi-fi choice review . I feared this when you mentioned these in your other post in November - but then you replied that you had already bought them .
{4} Perhaps M-S have identified a problem with this model as I see they have replaced it with their new 908 . This has a different cross-over type between the mid-bass drivers and the sub-woofer . I suggest you take your 502s to an M-S dealer and compare them to the 908 - or - perhaps a dealer will bring the 908s to try in your room - you must compare both in the same room and preferably both with the same amplifier to get an accurate comparison .
{5} If you got a "better" sound after you tried the hi-fi choice suggestions , or my suggestion in {3} above , then next try the speakers in different positions in your room , as some others have mentioned in this thread . Keep them out of corners , but try with them at different distances from the side walls than to the back wall - yes , this is very time consuming but you may find a position where the bass response is better than all other positions . This will be owing to standing waves in your room - a topic too complex for me to explain quickly here .
{6} If you are still not happy after all that , then I think the problem will be that the speakers are for use with an AV processor and NOT for only 2-channel music {stereo} . I would forget all about speakers with sub-woofers with built in amplifiers , etc ... as these are diificult to design and get satisfactory for simple stereo music except in some very expensive models . So if you are still not happy after trying all the ideas above then I suggest you trade in your speakers on the Mordaunt-Short 908 but only if you really like the 908 .
Before you go for the 908 comparison do listen to a good basic 3-way floorstanding speaker , of which the cheapest that I know of that will work with your Marantz receiver to sufficient volume in a large room is the B&W 704 . Listen also to the next price up B&W 703 - it sounds different to the 704 and you may prefer it . If you can afford more money then listen to the more expensive B&W 804 and 803 and 802 . After you have heard the B&W models that you can afford then listen to some other brands at that price and to the M-S 908 if you can afford it {I haven't looked at its price} . The B&W designs are all sensible designs that will work for 2-channel sound , so use them as a starting point to get an idea of "stereo" sound instead of THX / AV , etc ... best wishes , Chris .