as far as I know, you can just use Multi channel PCM, for DTS-HD and Dolby True HD audio signals.

Seems like your Roku 2 box, wont support multi channel PCM output perhaps, and I would advise you to replace this player then.
For example the Apple TV 4 and later will support Multi channel PCM just fine, aswell as an cheap raspberri pi box with Kodi.

So Pro Logic II is I think perhaps the limit for your Roku 2 device, or try bitstream mode with DTS/DD if it supports that.

#1 568.2mm, HD621, G91DH, 513, 511, DSP3100 , DSP33C , DSP33, 2x MJ Pro 50 MKI, M1500
#2 G68J, HD621, DSP3100, DSP5200HC, DSP33, 3x MJ Pro 50 MKII, MS200, SB Touch, Vertex 2
#3 G61rsl, HD621, 596P, DSP7000, DSP5500HC, DSP33, 2x MJ Pro 50 MKII, M1500

Applied mods: NOS-SRx Filter by Mr. Apodizer
Mr. Apodizer Digi box