If noise isolation is a high priority for you, the HT-21s aren't your best bet; although they technically have a closed-back design, the loose fit caused by the wide-angled headband structure unfortunately bleeds music to the outside world and doesn't do much to prevent peripheral noise from entering your ears. That said, a simple workaround is to simply adjust the volume to prevent sound leakage or raise it to drown out unwanted background noise, but this method is an easy way to damage your hearing over time.
Keep in mind that these are $40 headphones, so the HT-21s aren't in direct competition with high-end DJ headphones or professional quality monitors designed to expose your ears to undiscovered nuances in music. That being said, their overall sound signature is still a vast improvement over most of the stock earphones that come bundled with modern music players and we don't doubt your satisfaction with them, although certain sonic characteristics differentiate them from their direct competitors from Sennheiser and Koss.
Compared with the PortaPros, the most glaring distinction is the subdued attention to the low end. The HT-21s still emit a punchy beat but seem to cut back on the booming extension in favor of a more balanced mid and treble section. You can certainly add more bass emphasis using the equalizer on your music device, but you'll also lose the natural sound that puts MEElectronics in good standing with the mobile audiophile community. We also prefer the smoother bass control over the Sennheiser PX 100-IIi set's thudding quality that can get muddy and weighed down in hip-hop and electronic tracks that employ artificial drum machines.
On the other hand, the HT-21 headphones handle midrange and treble tones with crisp feedback that borders on the cooler end of the spectrum, sometimes giving off an unwelcome hiss in songs that are heavy with sibilant consonants (S and SH sounds). You can cut down these imperfections by lowering the listening volume through the headphones, although issue is easily ignored regardless.
The MEElectronics HT-21 headphones are suitable for anyone who doesn't want to spend a lot of money but still insists on a above-average sound without the retro flair of the Koss PortaPro or the smartphone-friendly remote on the Sennheiser PX 100-IIi. Sporting a comfortable design and a forward-thinking wire-and-plug combination, the HT-21 cans are a solid choice in the sub-$50 headphone market.