And Welcome
Able2know mod rewrite in itself is a lot less efficient than each one of the
phpBB SEO mod rewrites.
Because of the numerous duplicates it's allowing.
The phpBB SEO mod rewrites do not output any pagination duplicates and do only rewrite what is necessary (no advanced topic sorting and such). And they are a lot faster than the abl2know one.
If you add the
zero duplicate mod, you'll be amazed to run a 100% duplicate free web site

.
So this is a good reason to think about migrating to one of the phpBB seo mod rewrites (mixed or advanced would be my advise in your case).
Then, talking about the reason why Google is not really doing a good job on your forum, you're right,
1 single page outside the omitted results, with
10 600 listed URLs is just too bad.
You spotted correctly the reason for this, they all use the exact same meta tags, and if it's not as bad as if it was the title of the page to be static, it's still problematic.
Besides, PageRank was not transmitted to forums it seems.
So you can do several things, the first one being to take your time to properly define an SEO strategy.
The
dynamic meta tag mod will as well solve you're Google issue.
You could only use the meta tag mod together with
mx Google sitemaps module, as the mod is ready for able2know URLs as well, but, as said, you should migrate IMHO.
Reason to do so would be to get rid of all duplicates, which helps out a lot to show up better in SERPs, no more content dilution, no more content competition with itself.
And, this can be done without jeopardizing anything SEOwise thanks to the zero duplicate. The able2knwo cas started to be covered here :
http://www.phpbb-seo.com/boards/phpbb-m ... vt303.html
The pphpBB SEO web site is itself aan example of the HTTP 301 redirection effect on Search engines, both forum recently
went totally redirected without any penalty.
Steps are easy, once you've made up your mind about migrating and for which mod, please start a new thread and I'll help you out
Another thing you could do, is only install the dynamic meta tags for now, and wait a bit until the mod rewrite updates, which won't be too long from now. Teaser : -http://phpbbdev.phpbb-seo.net/ (look at the stats

)
As you can see if you compare with the
actual demo (take a look at the
demo settings page) I changed the URL standard a bit, -vt to -t etc ...
It's because the mod will offer for every one to choose a custom URL standard very easy, and as well because the zero duplicate is ready. So that first user will be able to keep or redirect the -vt and new one the -t.
Since your web site is quite big, you should either wait the update or directly install the old one with the new delimiters, which I'd help you out to do.
This only if you prefer the -t -f -c standard, and in order to only redirect only once all your URLs, if you go for migration of course.
This is quite some information, as said, take you time, you're not on a rush, install the dynamic meta tags, and think about you're URLs
++