Here is what I have:
Mypressi V2
Polrex tall grinder
Concept Art calibrated Tamper
Capresso H20 Plus Glass Water Kettle
Sant' Eustachio Espresso Grind
Sant' Eustachio Whole Bean
Ryan Bros Coffee (San Diego local roaster) Espresso Gold Whole Bean

I can pull 25~seconds espresso shot from both Sant' Eustachio & Espresso Gold by using 32lb calibrated tamper.

However, both espresso are too bitter. I know that mypressi temperature won't reach high enough to make espresso bitter, and I do get lots of crema from fresh bean (Ryan Bros Coffee) comparing to Sant' Eustachio (little crema). What else will cause the espresso bitter? I remember when I was in Rome, Italy, I tasted Sant' Eustachio, and it was so good, I don't remember bitterness (like I make at home).

Any help would greatly appreciated.

Shawn

No crema, or barely any crema, is generally stale coffee.

Do the pulls start out watery black, change to a crema-ish color halfway through, but keep that watery consistency?

Probably stale coffee then. Coffee that isn't fresh makes very bitter harsh espresso.

Some coffees are bitter when under extracted, its a woody bitterness as opposed to a tarry bitterness when over extracted.

Or it could be the roast of the coffee. Poorly roasted coffee or stale coffee can have all kinds of bad flavors. KlatchRoasting is on your side of the continent. Try a coffee from them and see if you have the same result.

Thanks for the suggestion.
However, Klatch Roasting has so many different kind of the blend, which one should I get in order to compare?

Thanks again!

Make the decision based on the flavour profile you think you'll enjoy the most. For instance the House Blend and the Belle Espresso sound great! You're essentially comparing stale (stuff you've used) to fresh coffee anyway.