[kylekohlmorgen]

I have some heavy toast oak cubes laying around...

Most sources will suggest using french oak with medium toast. What differences can I expect?

[ryang]

in general, expect a "heavier" presence.  more "oaky", toasty -perhaps a bit of char roast, quite carmelly.  I have had good success blending heavy with medium toast.  heavy on it's own is not quite my thing (when used at equal doses as lighter toasts).  I like oak to complement and not dominate.  that being said, you could use strictly heavy toast, just perhaps scaled back a bit more than you would dose with say house, or medium toast.

[enso]

As with any oak cubes you can mellow them a bit by soaking in wine or beer and dumping the soaking liquid out a few times.  That will extract some of the flavor each time you do it.  Or even doing the same with boiling water.  You will still have the heavier char/toast character but you can reduce its potency.

What were you planning on using them in?

[kylekohlmorgen]

They will be going into a RIS I made this spring

[redbeerman]

I would let them set for at leat 5 weeks.  Give it a taste and go from there.  The flavor will mellow over time after you pull them out.  I oaked an RIS with Hungarian medium toast and it created a very complex vanilla/oak flavor after 12 weeks.  It has been off the oak for about 8 weeks and I find the flavor still developing.  I brewed this beer about a year ago.

[1vertical]

I am digging the thing that my Hungarian oak does...not sure what the toast was
methinks Medium.