Good Morning Shrrrk,
Thank you for your questions.
We always advise to place some sheets of MDF or ply wood over a slatted base if the gaps are bigger than 3″. If they aren’t then there is no need to do this. The reason for the MDF or ply wood is that that mattress will attempt to settle within those gaps causing damage.
When any pocket sprung mattress goes on to a solid base then the spring tension inside will be firmed up. That top upholstery does not change (The comfort layer is a consistent feel and sensation). A sprung base will always soften down those springs inside the mattress.
There is a degree of logic in what both Advisor’s have told you; I would say that the firmest feel would be the mattress on the floor because with the solid slats you have the gaps between them providing a bit of ‘give’.
I hope this has answered your questions fully but if you do have any further queries then please don’t hesitate to contact me on 0161 437 4419. Our office is open Monday to Friday 8am to 8pm and Saturday and Sunday 10am to 4pm.
Kind Regards, Lee.