I could be wrong but I believe that standing calf raises hit the gastrocs more and the seated ones work the soleus harder. If that is indeed the case, then I would switch the high volume work to the seated calf raise since the soleus is usually slow twitch dominant and use the heavy low-rep work on the standing raise because the gastroc is fast twitch dominant.
Chad Waterbury also recommends a dorsiflexion exercise to keep the calves growing. There are machines for this or you can sit on a bench with your legs extended and "curl" a dumbell with your toes, that is, flex your feet up towards you so you feel it in the front of your shin.
Also, incline walking of course.