If one has a good quality pre-amp and bypasses the built-in pre-amps in the interface - does it matter if the interface is high-end (expensive) or will a cheaper one do the same job providing its pre-amps are not being used?
First, quality is quality. A good quality interface will have not just good pre-amps, but also good DAC and ADCs, good quality headphone amps, etc...
Having said that. We're talking home recording here. If you're lucky enough to be able to budget a $1000 tube pre for your microphone, it's likely to do a lot more for your sound than ramping your 2i2 up to an Apogee, Rane or other high-end interface (not that those wouldn't hurt as they have pretty high quality pre's).
Bottom line, they will do the same job, but your converters and hp amps will be the same quality they were.
Which interface and which pre-amp are we talking? Maybe there are some here that have experience with them and can advise.
If your interface doesn't have channels with line inputs, you probably can't bypass the preamps.
Most of the mic/line inputs go through the preamp, the line input just having a pad to get the signal down to mic level. (or a level control that lets you turn down the preamp far enough to accept a line signal.
It's not a quality issue, it's a configuration issue. If you want to use external preamps, you should buy an interface with dedicated line inputs.
I see the interface you're using has combo XLR/TRS jacks. Check the manual and see if the TRS bypasses the pre's. If so, then you can use external devices without going through the I/F's pres. Upgrading the interface would probably be your best bet. A $1000 pre through a $200 interface won't get you very far.
If your interface has line input points and you put external preamps in front of them, you're using the interface as converters.
If you want to google how high or low quality an interface is for that intended use, 'converters' is the word to use.
Some people swear blind that converter quality makes a huge difference to your recorded sound; Others say it's the last thing you should worry about.