You don't mention what you are using at the moment, so it's impossible to say what would be an upgrade.
You mention USB input so are you actually meaning a DAC and headphone amp rather than just the amp?
You then mention the red and white inputs, which are line in and are the standard input for all amps.
If it's only an amp you want then the Schiit Magni 2 Uber is a steal at £125
If you need a DAC too then the Schiit Modi 2 Uber has USB, co-axial and optical inputs and should be around the same price.