Siblings in OSD's Black series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Bass | B | OSD Black Series SC800D digs deeper - 40Hz against 100Hz. |
| Amplifier match | B | OSD Black Series SC800D is the easier load at 8 ohms; the other is 4 ohms and wants an amplifier rated for it. |
| Size | depends | OSD Black T55E MKII is easier to place; OSD Black Series SC800D is bigger, which usually goes with fuller bass. Your room decides. |
| Type | depends | A Passive Surround against a Passive In-wall - a different job rather than a better one. |
| Cabinet | depends | Sealed vs In-wall. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Price | B | OSD Black Series SC800D is cheaper - $300 against $400, about 25% less. |
A OSD Black T55E MKII B OSD Black Series SC800D
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both speakers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
OSD Black Series SC800D is the easier recommendation: it leads on most of the measured areas and costs less. Take OSD Black T55E MKII if its size, looks or fit in your room suit you better. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 100Hz anyway, so judge these on the other rows instead. In a large or open-plan room, OSD Black Series SC800D has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, OSD Black T55E MKII is the easier speaker to get right.
And remember what a spec sheet cannot show: how each looks in your room and whether the finish suits it, what dealer support and warranty are like where you live, and how they actually sound to you. If you can hear them first - ideally with your own music - that will tell you more than any number above.