If you want to watch live "off-air" TV(which includes the local channels not carried by DIRECTV),then yes,you'd need an AM21 on each receiver that you plan on watching the local channels with? Do you really want to watch local channels not carried by D* on ALL of your receivers(meaning would you really want a total of 5 AM21's)?
but can't you just hook up the USB then plug both the H25 and the AM21 in? I know you can't use the pigtail power cord to jump from the AM21 to say a R22 but if both are plugged in to the wall it should work. Solidsignal says it will work with the H25
Dual ATSC tuners
Simple USB connection
Small and lightweight design
Works with the H25. (Will need to reset H25 after adding)