Hi dear7,
I don't know anything about Tc helicon, but I'm using Digit V2, so I think I can share some ideas with you.
Digit. V2 has basic functions like Compressor, Reverb, Enhance, Harmony mix, and each of those functions also has 3 mini functions to play around with. Able to use with Condenser mic or Dynamic mic too,
I'm using it because I no longer want to be in any band or with any other (too lazy to practice with others). I'm now a "One man band" type so I will occasionally need some fake harmony to avoid monotony or even boring for the audience.
So far I'm happy with it, but of course nothing will replace the real human voice.
Here's my sample that you may have more idea if you want to watch it;
The reason Robin Gibbs used his hand to cover his ear; He said it helped him to block off and not getting influent by any other sound of any other instruments. In other word he blocked himself from any other sound. It's almost the same reason when we use the headphones when we do the recording.
I would suggest you try to use the ear plugs and try to harmonize with others just to prevent yourself from hearing the others while you do the harmornizing. Try it, it might help.
When I was in my last rock band (THANKS) in BKK in early 70s, I was very lucky to have at least 4 of my bandmates that could do the harmonizing very well, essentially because 3 of us were also the lead singers ( Rewat Buddhinan, Krit Choktippatana, and I) so not much problem for each of us to sing our own harmonize notes!
We played/sang quite a lot of "Crosby, Stills, Nash & Young", if you don't know that group you can find some of their songs at YouTube and you may amaze at their harzmonizing ability (I'd suggest "Judy blue eyes suite" or "Carry on")
Also I'd suggest you to practice by just sing your harmonize part alone while playing/practice alone all by yourself. Get it downpack even if it may sound funny without other harmonize parts. If 10 takes don't do it then take another 20 takes. DO NOT GIVE UP, it may take you longer than you think, but just like riding bicycle you will not forget and you will getter better everytime. Trust this old man, it worked for me and it should work for you too.
BTW; thanks for your comment on my singing tone, I guess it's down to practice, practice and practice again! And I'm pretty sure you'll get a good laugh if I try to sing Thai song just because I've never try to practice singing Thai songs! 55555