dude the 404 does that perfectly, just build your sequences through resampling and there's no grid at all. the sp series in general is probably best for what you're looking for, i've used a mpc 1000, sampletrak, microsampler, and various sps (got a 505 coming in a few days as my 404 went on the fritz and i want more of a 303 sound.)
Besides the pad requirement, your asr10 would do a damn good job at this. If your jonzin for new piece of gear thou, any mpc would do, asrx, Roland sp, even zoom sampletrak
i personally wouldn't recommend the sampletrak for "non-grid" because only having 24 pads and 60 seconds makes it difficult to make sequences without the sequencer. i personally don't make beats using the resample method with it. it is a great sampler though especially for the 70 bucks i paid for it (love the sound.)
Any sampler really....You can always arm your daw to record, and play each track into it. You don't even need midi sync, since you can just edit the wave files on the spot.
Not necessarily the best way, but another option.