I'm not so sure a "severely" dished or warped LP can be played effectively on "any" vacuum table!Especially when there is a wide chasm of vinyl thicknesses to be factored in,and some Lp's just have more pliability than others.Some are just NOT going to "take".
In the case of my SOTA COSMOS series IV,which is extremely effective(in general)I have problems getting a good seal if there is "any" gap between the lip and LP.The slightest seperation of any part of the LP will not allow for a seal to be made.I can run my finger along the lip(or LP side),to attempt to push the lip enough to "coax" a good seal,but it's a hit or miss project.I'd say about a 70/30 chance of success.The 70% is the success rate,in my case.This occurs on "maybe" one out of thirty LP's,which will need a "finger job" -:).
Once it "takes",from successful finger pressure,it is perfect,but no guarantees here.Actually because some LP's have slight pressing "bumps" or high edge spots,this is problematic sometimes too.My answer to "those" little problems is to file down the problem,with a nail file.Easy,and rarely needed.
Still,this is rare,and all I'll do(if I have a badly warped or dished LP)is use a little "pinch clamp" to close the air gap in the hosing.This is quite safe,and easy to do.It fools the pump into "thinking" there is a seal(because in the case of the Cosmos,the pump automatically cycles "down" to a minimum "safe" pressure setting,so as not to potentially damage the vinyl)so I can play the LP anyway.Still a rare occurrence.
I've played around a little with the "FABULOUS"(I love the table) Basis,but I'd still be surprised if you can get a good seal on similar badly dished/warped LP's.You can pitch up the amount of vacuum,on the Basis,but I'd be afraid the amount of vacuum suction(at "that" point)could pose a safety issue with the "vinyl" itself.
Some feel the actual amount of vacuum pressure can negatively impact the "health" of the vinyl on Lp's.In over twenty years of using my vacuum systems,I've not had a single issue.NOT ONCE!!Or ticks and pops either.Just keep the platter clean.
A ring clamp is,from my experience,a terrible product,ergonomically!Sorry,but I've got a pal who "took out" two Lyra Titan i cartridges with a mere half second slip up.
It "can" happen,and I've seen the ruined results,from an unpredictable mishap!
I made a DIY flattener by using two 12 inch pieces of cut marble,and sticking very thin "cork shelf liner" to each inner side.The cork has a little "give" to it,so will not damage the grooves.The weight of the marble,along with a little heat should do the trick,but it's a learning process..
Just put the gizmo in the sun,for only about ten minutes(this needs experimentation,to guage how each LP will react)....I added a few felt pads to the bottom of one marble slab,and place the top piece,carefully in place.This looks quite nice in any room decor,and the entire shebang cost me about ten bucks!It looks a little like a thin lap-top computer when closed.
Yet,I'd still prefer to go the easy way,and spring for the Furutech.I'm just not spending that kind of money on it,especially as the price has gone up in the U.S.Although I have a friend,in Canada,who is a huge LP collector,and claims superb results!He loves it!!
Best
