Maybe a female voice, a bit quieter (the soundscapes are almost completely silent for me) and maybe add some high-room-size, long decay (5-10, maybe even 20 seconds), wide panned (like 100%) and moderately diffused (maybe 10-20%) reverb to the voice with like 30% mix or so, which would add a very airy tone and help the voice blend in a bit. If the TTS engine has a whisper setting (many do), add just a bit. It'll help thicken the reverb.
That, paired with bass-heavy soundscapes, will create a very nice balance between the low registers and the voice's high registers.
Just a thought. :)