I don't know about the HD-A20 but the most crucial differences btw the A2 & XA2 is the XA2 has a native 1080p with v.1.3 HDMI. The A2 only outputs 1080i and its HDMI is v.1.2. Additionally, the XA2 has 5.1 channel analog audio out, the A2 doesn't(this is a critical shortcoming for HD audio capability, DD+ & TrueHD). The only way you can get the benefits of DD+ and TrueHD on the A2 is through an HDMI AV connection. At present, HD DVD players will convert these higher resolution audio formats to multichannel PCM and transmit it on an HDMI connection along with the video. For you to hear this, your AVR or pre-pro must be capable of playing multichannel PCM audio received via HDMI. Some AV receivers & pre-pros with HDMI connections are merely switchers and can't handle HDMI audio.

Without this HDMI audio capability you'll be limited to hearing only the downconverted output over the optical connection (NOTE: The A2 eliminated the digital coaxil audio out). DD+ or TrueHD signals are decoded and converted to PCM in the player. This PCM signal is then re-encoded as a 1.5 Mbps DTS bitstream, which is then output over the Toslink connection and decoded yet again from DTS to (ultimately) analog by your AVR or pre/pro. This is bad news for those who are still without a DTS decoder in their AVR. They'll be stuck listening to downmixed analog two-channel output from the legacy RCA jacks.

If interested, the XA2 has bi-directional RS232 control, the A2 doesn't have RS232.