Hi guys

Thanks for giving so many advices and posting so many replys.

I actuall got the idea of playing bass trombone on tuba mouthpiece from either wikipedia or Edward Kleinhammer's book (we haveone in our band library). I cannot remember from which source I read that contrabass trombone players do use tuba mouthpieces. May be tuba mouthpieces are designed partly for countrabass trombones apart from tuba, but not bass trombones.
So I think I'll stick with my 1AL and work more onthe fundermentals.

For tuning, Our band instructor has actually bought the Bach chorales years back, but the band didn't (doesn't) seem to like it very much, frankly it's too slow for teenagers like us... He has stopped doing the chorales partly because of the poor feedback on his idea...
I'll try getting my sectionmates to play them on mouthpiece directly, playing it on instrument will very possibly bore them to death.
The main problem about tuning the section is that there's too many people in the section (for now, 6), even if you tune each person before playing, there's always people getting off-tune later. It's undebatably ultra-time-consuming to get a note in tune with 6 people who usually play different pitches all the time (sad but true). From the reponce to the Bach Chorales, I think they'll give a even more negative responce towards getting in-tune.

Anyway, I'll try my luck on my section for once and try getting my intonation correct in the first place.

By the way, what is your experience when you guys play low Bbs loud? From books I've read that low notes for trombones is naturally a sound of "rattling metal sheets", so is it also natural to crack at loud low Bbs too? Any general suggestion?

No, it's not natural to crack low pedal tones. What you need to do is get some recordings of bass trombone players - Doug Yeo, Randall Hawes, Blair Bollinger, Ben Van Dijk, just to name a few. They make some gorgeous sounds in the low register. They can play them loud when they need to, but they still sound very smooth and mellow. "Rattling metal sheets" is not the sound that I try to think of when playing in the low register. I don't know what book you might have read that from.

It all depends on the effect needed for the piece. "rattling metal sheets" sounds like a bad wording for a sometimes necessary effect. We do sometimes want that large, edgy, violent, fierce sound.

Check out things like: (Off the top of my head)
Prokofiev's Montagues and Capulets from Romeo and Juliet
Respighi's Pines of Rome and Fountains of Rome
Strauss's Introduction to Also Sprach Zarathustra
Berlioz' Hungarian March
Rossini's William Tell

Thanks guys I'll work harder on my fundermentals for the time being.