Hi,

 the help of delread4~ states the "delay time is at least one sample",
but it seems the lower limit is one block (and the lowest blocksize I
can use on linux seems to be 64 samples). I assume this is related to
the dsp scheduling order introducing one sample block latency.

If the delread~ is scheduled before the delwrite~, there will be one block of additional delay. You can force the delread~ to be scheduled after the delwrite~ by putting the two objects in different subpatches and connect them with dummy signals, as explained in 3.audio.examples/G05.execution.order.pd.

However, once you add a feedback path - as you do in your patch -, this trick doesn't work because in the digital domain, feedback without delay is simply not possible. The only thing you can do is decrease the block size. This is explained in 3.audio.examples/G04.control.blocksize.pd.

Christof