I feel that I can't give you any numbers unless it was really looked into, only generalizations.
Any headphone output of a tablet or bluetooth module or any other device solely depends on the DAC (digital to analog converter) used in the project in regards to sound quality. however Bluetooth is going to have a harder time due to Bluetooth data throughput limitations. Bluetooth 2.0/2.1 has a thoughput of like 2mbps, which is cutting is cutting it close. I may be wrong here, but if you have a compressed mp3 at 320kbps , the computer then uncompresses it, into like raw bitrate similar to a .wav file, (this is where it would be streamed over bluetooth) where it is then put into the DAC which makes it into the analog voltages you can hear. WAV is typically like 1411kbps, which is really close to that 2000kbps bluetooth ceiling. All of a sudden you have a little interference limiting data throughput and you have a dip/obvious errors in SQ.
You're at the mercy of the DAC in either situation. It tends to be a you-get-what-you-pay-for. But beware of audiophile-fluff. Any cheap tablet or Bluetooth module is going to have a cheap DAC to keep costs low. I would assume more media-centric tablets (like what the Archos was) would have a higher end DAC in it. Some tablets with their USB hosts can support an external DAC, but last I've heard android OS has issues assigning it properly. I have seen HDMI outputs be utilized for audio output, as HDMI carries the uncompressed digital signal which is fed into the receiving end's DAC. But anything with an HDMI tag is generally expensive due to licensing, but is probably your best option if you strive for the good stuff.
If you have the chance you should use something similar to an orchestral track, or something with complex mids and highs. Listen for differences between a phone and maybe a computer with a sound card. Heck if a DAC is garbage you'll hear it in the bass too.
An EQ will not make up for sound quality that would be lost in a poor DAC. EQs are more for flattening response. Equalizing/normalizing speaker/environmental peaks etc.
... to answer your original question, I'd stick with the headphone jack of the tablet.
I could be , but I think I'm correct in my statements.